diff --git a/src/rpc/avalanche.cpp b/src/rpc/avalanche.cpp
--- a/src/rpc/avalanche.cpp
+++ b/src/rpc/avalanche.cpp
@@ -278,9 +278,13 @@
             iscbparam.isNull() ? false : iscbparam.get_bool();
         CKey key = DecodeSecret(find_value(stake, "privatekey").get_str());
 
+        if (!key.IsValid()) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid private key");
+        }
+
         if (!pb.addUTXO(utxo, amount, uint32_t(height), iscoinbase,
                         std::move(key))) {
-            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid private key");
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Duplicated stake");
         }
     }
 
diff --git a/test/functional/abc_rpc_buildavalancheproof.py b/test/functional/abc_rpc_buildavalancheproof.py
--- a/test/functional/abc_rpc_buildavalancheproof.py
+++ b/test/functional/abc_rpc_buildavalancheproof.py
@@ -82,6 +82,12 @@
                                         [invalid_privkey],
                                         )
 
+        duplicate_stake = [good_stake] * 2
+        check_buildavalancheproof_error(-8,
+                                        "Duplicated stake",
+                                        duplicate_stake,
+                                        )
+
         self.log.info("Happy path")
         assert node.buildavalancheproof(0, 0, proof_master, [good_stake])