fix: prevent duplicate signatures in _signature_map

Signed-off-by: Mohit Yadav <ymohit799057@gmail.com>

Author: mohityadav8

src/hiero_sdk_python/transaction/transaction.py

@@ -192,8 +192,12 @@ def sign(self, private_key: PrivateKey) -> Transaction:
             # We initialize the signature map for this body_bytes if it doesn't exist yet
             self._signature_map.setdefault(body_bytes, basic_types_pb2.SignatureMap())
 
-            # Append the signature pair to the signature map for this transaction body
-            self._signature_map[body_bytes].sigPair.append(sig_pair)
+            # deduplication check
+            already_signed = any(sp.pubKeyPrefix == public_key_bytes for sp in self._signature_map[body_bytes].sigPair)
+
+            # append only if not already signed
+            if not already_signed:
+                self._signature_map[body_bytes].sigPair.append(sig_pair)
 
         return self
 

tests/unit/test_signature_deduplication.py

@@ -0,0 +1,21 @@
+from __future__ import annotations
+
+from hiero_sdk_python.crypto.private_key import PrivateKey
+from hiero_sdk_python.token.token_mint_transaction import TokenMintTransaction
+
+
+def test_multiple_keys_still_work():
+    tx = TokenMintTransaction()
+
+    key1 = PrivateKey.generate_ed25519()
+    key2 = PrivateKey.generate_ed25519()
+
+    tx.freeze()
+
+    tx.sign(key1)
+    tx.sign(key2)
+
+    body_bytes = next(iter(tx._signature_map.keys()))
+    sig_pairs = tx._signature_map[body_bytes].sigPair
+
+    assert len(sig_pairs) == 2