diff --git a/lib_embedded_protocol_alpha/src/apply.ml b/lib_embedded_protocol_alpha/src/apply.ml
index 2e2344081..92f66b61c 100644
--- a/lib_embedded_protocol_alpha/src/apply.ml
+++ b/lib_embedded_protocol_alpha/src/apply.ml
@@ -13,6 +13,7 @@ open Tezos_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 int (* `Permanent *)
 type error += Bad_contract_parameter of Contract.t * Script.expr option * Script.expr option (* `Permanent *)
 type error += Too_many_faucet
 
@@ -60,6 +61,16 @@ let () =
     (function Bad_contract_parameter (c, expected, supplied) ->
        Some (c, expected, supplied) | _ -> None)
     (fun (c, expected, supplied) -> Bad_contract_parameter (c, expected, supplied)) ;
+  register_error_kind
+    `Permanent
+    ~id:"operation.duplicate_endorsement"
+    ~title:"Duplicate endorsement"
+    ~description:"Two endorsements received for the same slot"
+    ~pp:(fun ppf k ->
+        Format.fprintf ppf "Duplicate endorsement for slot %d." k)
+    Data_encoding.(obj1 (req "slot" uint16))
+    (function Duplicate_endorsement k -> Some k | _ -> None)
+    (fun k -> Duplicate_endorsement k);
   register_error_kind
     `Temporary
     ~id:"operation.too_many_faucet"
@@ -79,7 +90,11 @@ let apply_delegate_operation_content
       fail_unless
         (Block_hash.equal block pred_block)
         (Wrong_endorsement_predecessor (pred_block, block)) >>=? fun () ->
+      fail_when
+        (endorsement_already_recorded ctxt slot)
+        (Duplicate_endorsement (slot)) >>=? fun () ->
       Baking.check_signing_rights ctxt slot delegate >>=? fun () ->
+      let ctxt = record_endorsement ctxt slot in
       let ctxt = Fitness.increase ctxt in
       Baking.pay_endorsement_bond ctxt delegate >>=? fun (ctxt, bond) ->
       Baking.endorsement_reward ~block_priority >>=? fun reward ->
diff --git a/lib_embedded_protocol_alpha/src/raw_context.ml b/lib_embedded_protocol_alpha/src/raw_context.ml
index 0405949d1..4f9200b7e 100644
--- a/lib_embedded_protocol_alpha/src/raw_context.ml
+++ b/lib_embedded_protocol_alpha/src/raw_context.ml
@@ -7,6 +7,8 @@
 (*                                                                        *)
 (**************************************************************************)
 
+module Int_set = Set.Make (Compare.Int)
+
 type t = {
   context: Context.t ;
   constants: Constants_repr.constants ;
@@ -16,7 +18,9 @@ type t = {
   fitness: Int64.t ;
   roll_value: Tez_repr.t ;
   faucet_count: int;
+  endorsements_received: Int_set.t;
 }
+
 type context = t
 type root_context = t
 
@@ -29,6 +33,9 @@ let constants ctxt = ctxt.constants
 let roll_value ctxt = ctxt.roll_value
 let recover ctxt = ctxt.context
 
+let record_endorsement ctxt k = { ctxt with endorsements_received = Int_set.add k ctxt.endorsements_received }
+let endorsement_already_recorded ctxt k = Int_set.mem k ctxt.endorsements_received
+
 let incr_faucet_count ctxt = { ctxt with faucet_count = ctxt.faucet_count + 1 }
 let set_current_fitness ctxt fitness = { ctxt with fitness }
 
@@ -228,7 +235,7 @@ let prepare ~level ~timestamp ~fitness ctxt =
       level in
   return ({ context = ctxt ; constants ; level ;
             timestamp ; fitness ; first_level ; roll_value ;
-            faucet_count = 0 ;
+            faucet_count = 0 ; endorsements_received = Int_set.empty ;
           },
           first_block)
 
@@ -257,6 +264,7 @@ let register_resolvers enc resolve =
       fitness = 0L ;
       roll_value = Tez_repr.zero ;
       faucet_count = 0 ;
+      endorsements_received = Int_set.empty ;
     } in
     resolve faked_context str in
   Context.register_resolver enc  resolve
diff --git a/lib_embedded_protocol_alpha/src/raw_context.mli b/lib_embedded_protocol_alpha/src/raw_context.mli
index 4023011e1..19494fdd3 100644
--- a/lib_embedded_protocol_alpha/src/raw_context.mli
+++ b/lib_embedded_protocol_alpha/src/raw_context.mli
@@ -134,6 +134,10 @@ end
 
 include T with type t := t and type context := context
 
+val record_endorsement: context -> int -> context
+val endorsement_already_recorded: context -> int -> bool
+
+
 (** HACK alphanet *)
 val double_roll_value: context -> int -> context tzresult Lwt.t
 val faucet_count: context -> int
diff --git a/lib_embedded_protocol_alpha/src/tezos_context.ml b/lib_embedded_protocol_alpha/src/tezos_context.ml
index a7b479ba1..474990bec 100644
--- a/lib_embedded_protocol_alpha/src/tezos_context.ml
+++ b/lib_embedded_protocol_alpha/src/tezos_context.ml
@@ -153,3 +153,6 @@ let fork_test_network = Raw_context.fork_test_network
 
 let faucet_count = Raw_context.faucet_count
 let incr_faucet_count = Raw_context.incr_faucet_count
+
+let endorsement_already_recorded = Raw_context.endorsement_already_recorded
+let record_endorsement = Raw_context.record_endorsement
diff --git a/lib_embedded_protocol_alpha/src/tezos_context.mli b/lib_embedded_protocol_alpha/src/tezos_context.mli
index eca1b515d..1621945f7 100644
--- a/lib_embedded_protocol_alpha/src/tezos_context.mli
+++ b/lib_embedded_protocol_alpha/src/tezos_context.mli
@@ -726,6 +726,9 @@ val configure_sandbox:
 val activate: context -> Protocol_hash.t -> context Lwt.t
 val fork_test_network: context -> Protocol_hash.t -> Time.t -> context Lwt.t
 
+val endorsement_already_recorded: context -> int -> bool
+val record_endorsement: context -> int -> context
+
 (**/**)
 
 (* HACK alphanet *)