diff --git a/solidity/contracts/domains/pente/PentePrivacyGroup.sol b/solidity/contracts/domains/pente/PentePrivacyGroup.sol
index 8bb4f47a5..95cb9c840 100644
--- a/solidity/contracts/domains/pente/PentePrivacyGroup.sol
+++ b/solidity/contracts/domains/pente/PentePrivacyGroup.sol
@@ -29,11 +29,13 @@ contract PentePrivacyGroup is IPente, UUPSUpgradeable, EIP712Upgradeable {
 
     mapping(bytes32 => bool) private _unspent;
     mapping(bytes32 => address) private _approvals;
+    mapping(bytes32 => bool) private _txids;
     address _nextImplementation;
 
     // Config follows the convention of a 4 byte type selector, followed by ABI encoded bytes
     bytes4 public constant PenteConfig_V0 = 0x00010000;
 
+    error PenteDuplicateTransaction(bytes32 txId);
     error PenteUnsupportedConfigType(bytes4 configSelector);
     error PenteDuplicateEndorser(address signer);
     error PenteInvalidEndorser(address signer);
@@ -155,6 +157,13 @@ contract PentePrivacyGroup is IPente, UUPSUpgradeable, EIP712Upgradeable {
         States calldata states,
         ExternalCall[] calldata externalCalls
     ) internal {
+
+        // On-chain enforcement of TXID uniqueness
+        if (_txids[txId] != false) {
+            revert PenteDuplicateTransaction(txId);
+        }
+        _txids[txId] = true;
+
         // Perform the state transitions
         for (uint i = 0; i < states.inputs.length; i++) {
             if (!_unspent[states.inputs[i]]) {
diff --git a/solidity/test/domains/pente/PentePrivacyGroup.ts b/solidity/test/domains/pente/PentePrivacyGroup.ts
index 3e53efe6c..96a55dadb 100644
--- a/solidity/test/domains/pente/PentePrivacyGroup.ts
+++ b/solidity/test/domains/pente/PentePrivacyGroup.ts
@@ -129,15 +129,16 @@ describe("PentePrivacyGroup", function () {
     );
     const tx1ID = randBytes32();
 
+    const tx1 = {
+      inputs: [],
+      reads: [],
+      outputs: stateSet1,
+      info: info1,
+    };
     await expect(
       privacyGroup.transition(
         tx1ID,
-        {
-          inputs: [],
-          reads: [],
-          outputs: stateSet1,
-          info: info1,
-        },
+        tx1,
         [],
         endorsements1
       )
@@ -145,6 +146,16 @@ describe("PentePrivacyGroup", function () {
       .to.emit(privacyGroup, "PenteTransition")
       .withArgs(tx1ID, [], [], stateSet1, info1);
 
+    // Rejects duplicate
+    await expect(
+      privacyGroup.transition(
+        tx1ID,
+        tx1,
+        [],
+        endorsements1
+      )
+    ).to.be.rejectedWith("PenteDuplicateTransaction");
+
     const stateSet2 = [randBytes32(), randBytes32(), randBytes32()];
     const inputs2 = [stateSet1[1]];
     const reads2 = [stateSet1[0], stateSet1[2]];