diff --git a/src/proto_alpha/lib_protocol/src/alpha_context.mli b/src/proto_alpha/lib_protocol/src/alpha_context.mli
index 8ddb00668..8ff42b23e 100644
--- a/src/proto_alpha/lib_protocol/src/alpha_context.mli
+++ b/src/proto_alpha/lib_protocol/src/alpha_context.mli
@@ -928,7 +928,7 @@ module Operation : sig
   type error += Invalid_signature (* `Permanent *)
 
   val check_signature: public_key -> _ operation -> unit tzresult Lwt.t
-  val raw_check_signature: public_key -> _ operation -> unit tzresult
+  val check_signature_sync: public_key -> _ operation -> unit tzresult
 
   val internal_operation_encoding: packed_internal_operation Data_encoding.t
 
@@ -1059,10 +1059,10 @@ val record_endorsement:
   context -> Signature.Public_key_hash.t -> context
 val allowed_endorsements:
   context ->
-  (Signature.Public_key.t * int list) Signature.Public_key_hash.Map.t
+  (Signature.Public_key.t * int list * bool) Signature.Public_key_hash.Map.t
 val init_endorsements:
   context ->
-  (Signature.Public_key.t * int list) Signature.Public_key_hash.Map.t ->
+  (Signature.Public_key.t * int list * bool) Signature.Public_key_hash.Map.t ->
   context
 
 val reset_internal_nonce: context -> context
diff --git a/src/proto_alpha/lib_protocol/src/apply.ml b/src/proto_alpha/lib_protocol/src/apply.ml
index b44fd5b9e..9b5d8f5ce 100644
--- a/src/proto_alpha/lib_protocol/src/apply.ml
+++ b/src/proto_alpha/lib_protocol/src/apply.ml
@@ -13,6 +13,7 @@ open Alpha_context
 
 type error += Wrong_voting_period of Voting_period.t * Voting_period.t (* `Temporary *)
 type error += Wrong_endorsement_predecessor of Block_hash.t * Block_hash.t (* `Temporary *)
+type error += Duplicate_endorsement of Signature.Public_key_hash.t (* `Branch *)
 type error += Invalid_endorsement_level
 type error += Invalid_commitment of { expected: bool }
 type error += Internal_operation_replay of packed_internal_operation
@@ -70,6 +71,17 @@ let () =
                      (req "provided" Voting_period.encoding))
     (function Wrong_voting_period (e, p) -> Some (e, p) | _ -> None)
     (fun (e, p) -> Wrong_voting_period (e, p));
+  register_error_kind
+    `Branch
+    ~id:"operation.duplicate_endorsement"
+    ~title:"Duplicate endorsement"
+    ~description:"Two endorsements received from same delegate"
+    ~pp:(fun ppf k ->
+        Format.fprintf ppf "Duplicate endorsement from delegate %a (possible replay attack)."
+          Signature.Public_key_hash.pp_short k)
+    Data_encoding.(obj1 (req "delegate" Signature.Public_key_hash.encoding))
+    (function Duplicate_endorsement k -> Some k | _ -> None)
+    (fun k -> Duplicate_endorsement k);
   register_error_kind
     `Temporary
     ~id:"operation.invalid_endorsement_level"
@@ -597,25 +609,31 @@ let apply_contents_list
   : (context * kind contents_result_list) tzresult Lwt.t =
   match contents_list with
   | Single (Endorsement { block ; level }) ->
-      let current_level = (Level.current ctxt).level in
       fail_unless
         (Block_hash.equal block pred_block)
         (Wrong_endorsement_predecessor (pred_block, block)) >>=? fun () ->
+      let block = operation.shell.branch in
+      fail_unless
+        (Block_hash.equal block pred_block)
+        (Wrong_endorsement_predecessor (pred_block, block)) >>=? fun () ->
+      let current_level = (Level.current ctxt).level in
       fail_unless
         Raw_level.(succ level = current_level)
         Invalid_endorsement_level >>=? fun () ->
-      Baking.check_endorsement_rights ctxt operation >>=? fun (delegate, slots) ->
-      let ctxt = record_endorsement ctxt delegate in
-      let gap = List.length slots in
-      let ctxt = Fitness.increase ~gap ctxt in
-      Lwt.return
-        Tez.(Constants.endorsement_security_deposit ctxt *?
-             Int64.of_int gap) >>=? fun deposit ->
-      add_deposit ctxt delegate deposit >>=? fun ctxt ->
-      Global.get_last_block_priority ctxt >>=? fun block_priority ->
-      Baking.endorsement_reward ctxt ~block_priority gap >>=? fun reward ->
-      Delegate.freeze_rewards ctxt delegate reward >>=? fun ctxt ->
-      return (ctxt, Single_result (Endorsement_result (delegate, slots)))
+      Baking.check_endorsement_rights ctxt operation >>=? fun (delegate, slots, used) ->
+      if used then fail (Duplicate_endorsement delegate)
+      else
+        let ctxt = record_endorsement ctxt delegate in
+        let gap = List.length slots in
+        let ctxt = Fitness.increase ~gap ctxt in
+        Lwt.return
+          Tez.(Constants.endorsement_security_deposit ctxt *?
+               Int64.of_int gap) >>=? fun deposit ->
+        add_deposit ctxt delegate deposit >>=? fun ctxt ->
+        Global.get_last_block_priority ctxt >>=? fun block_priority ->
+        Baking.endorsement_reward ctxt ~block_priority gap >>=? fun reward ->
+        Delegate.freeze_rewards ctxt delegate reward >>=? fun ctxt ->
+        return (ctxt, Single_result (Endorsement_result (delegate, slots)))
   | Single (Seed_nonce_revelation { level ; nonce }) ->
       let level = Level.from_raw ctxt level in
       Nonce.reveal ctxt level nonce >>=? fun ctxt ->
@@ -639,8 +657,8 @@ let apply_contents_list
             (Outdated_double_endorsement_evidence
                { level = level.level ;
                  last = oldest_level }) >>=? fun () ->
-          Baking.check_endorsement_rights ctxt op1 >>=? fun (delegate1, _) ->
-          Baking.check_endorsement_rights ctxt op2 >>=? fun (delegate2, _) ->
+          Baking.check_endorsement_rights ctxt op1 >>=? fun (delegate1, _, _) ->
+          Baking.check_endorsement_rights ctxt op2 >>=? fun (delegate2, _, _) ->
           fail_unless
             (Signature.Public_key_hash.equal delegate1 delegate2)
             (Inconsistent_double_endorsement_evidence
diff --git a/src/proto_alpha/lib_protocol/src/apply_operation_result.ml b/src/proto_alpha/lib_protocol/src/apply_operation_result.ml
index 55aecb348..e8ac86403 100644
--- a/src/proto_alpha/lib_protocol/src/apply_operation_result.ml
+++ b/src/proto_alpha/lib_protocol/src/apply_operation_result.ml
@@ -386,7 +386,7 @@ module Encoding = struct
       encoding =
         (obj2
            (req "delegate" Signature.Public_key_hash.encoding)
-           (req "slots" (list uint8))) ;
+           (req "slots" (list uint8)));
       select =
         (function
           | Contents_result (Endorsement_result _ as op) -> Some op
diff --git a/src/proto_alpha/lib_protocol/src/baking.ml b/src/proto_alpha/lib_protocol/src/baking.ml
index 11123e822..93a012a1e 100644
--- a/src/proto_alpha/lib_protocol/src/baking.ml
+++ b/src/proto_alpha/lib_protocol/src/baking.ml
@@ -13,7 +13,7 @@ open Misc
 
 type error += Invalid_fitness_gap of int64 * int64 (* `Permanent *)
 type error += Timestamp_too_early of Timestamp.t * Timestamp.t (* `Permanent *)
-type error += Unexpected_endorsement
+type error += Unexpected_endorsement (* `Permanent *)
 type error += Invalid_block_signature of Block_hash.t * Signature.Public_key_hash.t (* `Permanent *)
 type error += Invalid_signature  (* `Permanent *)
 type error += Invalid_stamp  (* `Permanent *)
@@ -158,11 +158,11 @@ let endorsement_rights c level =
     (fun acc slot ->
        Roll.endorsement_rights_owner c level ~slot >>=? fun pk ->
        let pkh = Signature.Public_key.hash pk in
-       let slots =
+       let right =
          match Signature.Public_key_hash.Map.find_opt pkh acc with
-         | None -> (pk, [slot])
-         | Some (pk, slots) -> (pk, slot :: slots) in
-       return (Signature.Public_key_hash.Map.add pkh slots acc))
+         | None -> (pk, [slot], false)
+         | Some (pk, slots, used) -> (pk, slot :: slots, used) in
+       return (Signature.Public_key_hash.Map.add pkh right acc))
     Signature.Public_key_hash.Map.empty
     (0 --> (Constants.endorsers_per_block c - 1))
 
@@ -174,20 +174,17 @@ let check_endorsement_rights ctxt (op : Kind.endorsement Operation.t) =
       return (Alpha_context.allowed_endorsements ctxt)
     else
       endorsement_rights ctxt (Level.from_raw ctxt level)
-  end >>=? fun map ->
+  end >>=? fun endorsements ->
   match
-    Signature.Public_key_hash.Map.find_first_opt
-      (fun pkh ->
-         let pk, _ = Signature.Public_key_hash.Map.find pkh map in
-         match Operation.raw_check_signature pk op with
-         | Error _ -> false
-         | Ok () -> true)
-      map
+    Signature.Public_key_hash.Map.fold (* no find_first *)
+      (fun pkh (pk, slots, used) acc ->
+         match Operation.check_signature_sync pk op with
+         | Error _ -> acc
+         | Ok () -> Some (pkh, slots, used))
+      endorsements None
   with
-  | None ->
-      fail Unexpected_endorsement
-  | Some (pkh, (_pk, slots)) ->
-      return (pkh, slots)
+  | None -> fail Unexpected_endorsement
+  | Some v -> return v
 
 let select_delegate delegate delegate_list max_priority =
   let rec loop acc l n =
diff --git a/src/proto_alpha/lib_protocol/src/baking.mli b/src/proto_alpha/lib_protocol/src/baking.mli
index 8c659f0d3..2a7f25510 100644
--- a/src/proto_alpha/lib_protocol/src/baking.mli
+++ b/src/proto_alpha/lib_protocol/src/baking.mli
@@ -31,18 +31,19 @@ val check_baking_rights:
   context -> Block_header.contents -> Time.t ->
   public_key tzresult Lwt.t
 
-(** [check_endorsements_rights c slots]:
-    * verifies that the endorsement slots are valid ;
-    * verifies that the endorsement slots correspond to the same delegate at the current level;
-*)
-val check_endorsement_rights:
-  context -> Kind.endorsement Operation.t ->
-  (public_key_hash * int list) tzresult Lwt.t
-
+(** For a given level computes who has the right to
+    include an endorsement in the next block.
+    The result can be stored in Alpha_context.allowed_endorsements *)
 val endorsement_rights:
   context ->
   Level.t ->
-  (public_key * int list) Signature.Public_key_hash.Map.t tzresult Lwt.t
+  (public_key * int list * bool) Signature.Public_key_hash.Map.t tzresult Lwt.t
+
+(** Check that the operation was signed by a delegate allowed
+    to endorse at the level specified by the endorsement. *)
+val check_endorsement_rights:
+  context -> Kind.endorsement Operation.t ->
+  (public_key_hash * int list * bool) tzresult Lwt.t
 
 (** Returns the endorsement reward calculated w.r.t a given priotiry.  *)
 val endorsement_reward: context -> block_priority:int -> int -> Tez.t tzresult Lwt.t
diff --git a/src/proto_alpha/lib_protocol/src/delegate_services.ml b/src/proto_alpha/lib_protocol/src/delegate_services.ml
index 8607151c8..780b92484 100644
--- a/src/proto_alpha/lib_protocol/src/delegate_services.ml
+++ b/src/proto_alpha/lib_protocol/src/delegate_services.ml
@@ -460,7 +460,7 @@ module Endorsing_rights = struct
       RPC_service.get_service
         ~description:
           "Retrieves the delegates allowed to endorse a block.\n\
-           By default, it gives the endorsement slots for bakers that \
+           By default, it gives the endorsement slots for delegates that \
            have at least one in the next block.\n\
            Parameters `level` and `cycle` can be used to specify the \
            (valid) level(s) in the past or future at which the \
@@ -483,7 +483,7 @@ module Endorsing_rights = struct
     Baking.endorsement_rights ctxt level >>=? fun rights ->
     return
       (Signature.Public_key_hash.Map.fold
-         (fun delegate (_, slots) acc -> {
+         (fun delegate (_, slots, _) acc -> {
               level = level.level ; delegate ; slots ; estimated_time
             } :: acc)
          rights [])
@@ -529,4 +529,3 @@ let baking_rights ctxt max_priority =
           List.map
             (fun { Baking_rights.delegate ; timestamp ; _ } ->
                (delegate, timestamp)) l)
-
diff --git a/src/proto_alpha/lib_protocol/src/operation_repr.ml b/src/proto_alpha/lib_protocol/src/operation_repr.ml
index 6de73b3ee..1be4d3142 100644
--- a/src/proto_alpha/lib_protocol/src/operation_repr.ml
+++ b/src/proto_alpha/lib_protocol/src/operation_repr.ml
@@ -646,7 +646,7 @@ let () =
     (function Missing_signature -> Some () | _ -> None)
     (fun () -> Missing_signature)
 
-let raw_check_signature (type kind) key ({ shell ; protocol_data } : kind operation) =
+let check_signature_sync (type kind) key ({ shell ; protocol_data } : kind operation) =
   let check ~watermark contents signature =
     let unsigned_operation =
       Data_encoding.Binary.to_bytes_exn
@@ -668,7 +668,7 @@ let raw_check_signature (type kind) key ({ shell ; protocol_data } : kind operat
       check ~watermark:Generic_operation (Contents_list contents) signature
 
 let check_signature pk op =
-  Lwt.return (raw_check_signature pk op)
+  Lwt.return (check_signature_sync pk op)
 
 let hash_raw = Operation.hash
 let hash (o : _ operation) =
diff --git a/src/proto_alpha/lib_protocol/src/operation_repr.mli b/src/proto_alpha/lib_protocol/src/operation_repr.mli
index 0755a1e4d..8589afd7c 100644
--- a/src/proto_alpha/lib_protocol/src/operation_repr.mli
+++ b/src/proto_alpha/lib_protocol/src/operation_repr.mli
@@ -165,7 +165,7 @@ type error += Invalid_signature (* `Permanent *)
 
 val check_signature:
   Signature.Public_key.t -> _ operation -> unit tzresult Lwt.t
-val raw_check_signature:
+val check_signature_sync:
   Signature.Public_key.t -> _ operation -> unit tzresult
 
 
diff --git a/src/proto_alpha/lib_protocol/src/raw_context.ml b/src/proto_alpha/lib_protocol/src/raw_context.ml
index 699a24856..a0d7ec771 100644
--- a/src/proto_alpha/lib_protocol/src/raw_context.ml
+++ b/src/proto_alpha/lib_protocol/src/raw_context.ml
@@ -18,7 +18,7 @@ type t = {
   fitness: Int64.t ;
   deposits: Tez_repr.t Signature.Public_key_hash.Map.t ;
   allowed_endorsements:
-    (Signature.Public_key.t * int list) Signature.Public_key_hash.Map.t ;
+    (Signature.Public_key.t * int list * bool) Signature.Public_key_hash.Map.t ;
   fees: Tez_repr.t ;
   rewards: Tez_repr.t ;
   block_gas: Z.t ;
@@ -41,11 +41,23 @@ let constants ctxt = ctxt.constants
 let recover ctxt = ctxt.context
 
 let record_endorsement ctxt k =
-  { ctxt with
-    allowed_endorsements =
-      Signature.Public_key_hash.Map.remove k ctxt.allowed_endorsements }
+  match Signature.Public_key_hash.Map.find_opt k ctxt.allowed_endorsements with
+  | None -> assert false
+  | Some (_, _, true) -> assert false (* right already used *)
+  | Some (d, s, false) ->
+      { ctxt with
+        allowed_endorsements =
+          Signature.Public_key_hash.Map.add k (d,s,true) ctxt.allowed_endorsements }
+
 let init_endorsements ctxt allowed_endorsements =
-  { ctxt with allowed_endorsements }
+  if Signature.Public_key_hash.Map.is_empty allowed_endorsements
+  then assert false (* can't initialize to empty *)
+  else begin
+    if Signature.Public_key_hash.Map.is_empty ctxt.allowed_endorsements
+    then { ctxt with allowed_endorsements }
+    else assert false (* can't initialize twice *)
+  end
+
 let allowed_endorsements ctxt =
   ctxt.allowed_endorsements
 
diff --git a/src/proto_alpha/lib_protocol/src/raw_context.mli b/src/proto_alpha/lib_protocol/src/raw_context.mli
index 94b7b5771..7d3b8455a 100644
--- a/src/proto_alpha/lib_protocol/src/raw_context.mli
+++ b/src/proto_alpha/lib_protocol/src/raw_context.mli
@@ -205,12 +205,19 @@ val record_internal_nonce: context -> int -> context
 (** Check is the internal operation nonce has been taken. *)
 val internal_nonce_already_recorded: context -> int -> bool
 
-val record_endorsement:
-  context -> Signature.Public_key_hash.t -> context
+(** Returns a map where to each endorser's pkh is associated the list of its
+    endorsing slots (in decreasing order) for a given level. *)
 val allowed_endorsements:
   context ->
-  (Signature.Public_key.t * int list) Signature.Public_key_hash.Map.t
+  (Signature.Public_key.t * int list * bool) Signature.Public_key_hash.Map.t
+
+(** Initializes the map of allowed endorsements, this function must only be
+    called once. *)
 val init_endorsements:
   context ->
-  (Signature.Public_key.t * int list) Signature.Public_key_hash.Map.t ->
+  (Signature.Public_key.t * int list * bool) Signature.Public_key_hash.Map.t ->
   context
+
+(** Marks an endorsment in the map as used. *)
+val record_endorsement:
+  context -> Signature.Public_key_hash.t -> context