diff --git a/src/hiero_sdk_python/transaction/transaction.py b/src/hiero_sdk_python/transaction/transaction.py
index 692877c20..763aac655 100644
--- a/src/hiero_sdk_python/transaction/transaction.py
+++ b/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
 
diff --git a/tests/unit/transaction_test.py b/tests/unit/transaction_test.py
index efe8cbe84..5821d9e7a 100644
--- a/tests/unit/transaction_test.py
+++ b/tests/unit/transaction_test.py
@@ -3,6 +3,7 @@
 import pytest
 
 from hiero_sdk_python.account.account_create_transaction import AccountCreateTransaction
+from hiero_sdk_python.account.account_id import AccountId
 from hiero_sdk_python.crypto.private_key import PrivateKey
 from hiero_sdk_python.exceptions import ReceiptStatusError
 from hiero_sdk_python.hapi.services import (
@@ -14,6 +15,9 @@
     transaction_response_pb2,
 )
 from hiero_sdk_python.response_code import ResponseCode
+from hiero_sdk_python.tokens.token_id import TokenId
+from hiero_sdk_python.tokens.token_mint_transaction import TokenMintTransaction
+from hiero_sdk_python.transaction.transaction_id import TransactionId
 from hiero_sdk_python.transaction.transaction_receipt import TransactionReceipt
 from hiero_sdk_python.transaction.transaction_response import TransactionResponse
 from tests.unit.mock_server import mock_hedera_servers
@@ -117,3 +121,42 @@ def test_execute_returns_receipt_without_error_when_validation_disabled():
         receipt = tx.execute(client)
 
         assert receipt.status == ResponseCode.INVALID_SIGNATURE
+
+
+def test_duplicate_signature_not_added():
+    tx = TokenMintTransaction()
+    tx.set_transaction_id(TransactionId.generate(AccountId(0, 0, 1234)))
+    tx.set_node_account_id(AccountId(0, 0, 3))
+    tx.set_token_id(TokenId(0, 0, 1))
+    tx.set_amount(100)
+    key = PrivateKey.generate_ed25519()
+    tx.freeze()
+    tx.sign(key)
+    tx.sign(key)
+    assert tx._signature_map, "signature_map should not be empty after freeze+sign"  # ← ADD HERE
+    body_bytes = next(iter(tx._signature_map.keys()))
+    sig_pairs = tx._signature_map[body_bytes].sigPair
+    assert len(sig_pairs) == 1, "Expected 1 signature for duplicate key"
+
+
+def test_multiple_keys_still_work():
+    tx = TokenMintTransaction()
+    tx.set_transaction_id(TransactionId.generate(AccountId(0, 0, 1234)))
+    tx.set_node_account_id(AccountId(0, 0, 3))
+    tx.set_token_id(TokenId(0, 0, 1))
+    tx.set_amount(100)
+    key1 = PrivateKey.generate_ed25519()
+    key2 = PrivateKey.generate_ed25519()
+    tx.freeze()
+    tx.sign(key1)
+    tx.sign(key2)
+    assert tx._signature_map, "signature_map should not be empty after freeze+sign"
+    body_bytes = next(iter(tx._signature_map.keys()))
+    sig_pairs = tx._signature_map[body_bytes].sigPair
+    assert len(sig_pairs) == 2, "Expected 2 signatures for different keys"
+    pubkey_prefixes = {sp.pubKeyPrefix for sp in sig_pairs}
+    expected_prefixes = {
+        key1.public_key().to_bytes_raw(),
+        key2.public_key().to_bytes_raw(),
+    }
+    assert pubkey_prefixes == expected_prefixes, "Signatures should match key1 and key2 exactly"