Dedupe signatures in build_and_sign

Users can sometimes pass same signing keys, which leads to duplicated signatures, resulting the actual fee to be greater than estimated fee. This commit will remove deplicated signatures and therefore eliminate this type of issue.

pycardano/key.py

@@ -148,6 +148,9 @@ def __eq__(self, other):
     def __repr__(self) -> str:
         return self.to_json()
 
+    def __hash__(self):
+        return hash(self.payload)
+
 
 class SigningKey(Key):
     def sign(self, data: bytes) -> bytes:

pycardano/txbuilder.py

@@ -1256,7 +1256,7 @@ def build_and_sign(
         witness_set = self.build_witness_set()
         witness_set.vkey_witnesses = []
 
-        for signing_key in signing_keys:
+        for signing_key in set(signing_keys):
             signature = signing_key.sign(tx_body.hash())
             witness_set.vkey_witnesses.append(
                 VerificationKeyWitness(signing_key.to_verification_key(), signature)

test/pycardano/test_key.py

@@ -97,3 +97,18 @@ def test_key_save():
         SK.save(f.name)
         sk = PaymentSigningKey.load(f.name)
         assert SK == sk
+
+
+def test_key_hash():
+    sk = PaymentSigningKey.generate()
+    vk = PaymentVerificationKey.from_signing_key(sk)
+
+    sk_set = set()
+    vk_set = set()
+
+    for _ in range(2):
+        sk_set.add(sk)
+        vk_set.add(vk)
+
+    assert len(sk_set) == 1
+    assert len(vk_set) == 1