fix: prevent duplicate signatures in _signature_map

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

Author: mohityadav8

tests/unit/signature_deduplication_test.py

@@ -0,0 +1,44 @@
+# RENAME FILE TO: tests/unit/signature_deduplication_test.py
+
+from __future__ import annotations
+
+from hiero_sdk_python.account.account_id import AccountId
+from hiero_sdk_python.crypto.private_key import PrivateKey
+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
+
+
+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))
+    key = PrivateKey.generate_ed25519()
+    tx.freeze()
+    tx.sign(key)
+    tx.sign(key)
+    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))
+    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, "Expected 2 signatures for different keys"
+    pubkey_prefixes = {sp.pubKeyPrefix for sp in sig_pairs}
+    expected_prefixes = {
+        key1.public_key().to_bytes(),
+        key2.public_key().to_bytes(),
+    }
+    assert pubkey_prefixes == expected_prefixes, "Signatures should match key1 and key2 exactly"