Fix wrongly register PNA as ENA

contracts/src/Assets.sol

@@ -252,7 +252,11 @@ library Assets {
         // NOTE: Explicitly allow a token to be re-registered. This offers resiliency
         // in case a previous registration attempt of the same token failed on the remote side.
         // It means that registration can be retried.
+        // But register a PNA here is not allowed
         TokenInfo storage info = $.tokenRegistry[token];
+        if(info.foreignID != bytes32(0)) {
+            revert TokenAlreadyRegistered();
+        }
         info.isRegistered = true;
 
         ticket.dest = $.assetHubParaID;

contracts/test/Gateway.t.sol

@@ -1014,4 +1014,12 @@ contract GatewayTest is Test {
         bytes memory encodedParams = abi.encode(params);
         MockGateway(address(gateway)).agentExecutePublic(encodedParams);
     }
+
+    function testRegisterForeignTokenAsNativeTokenWillFail() public {
+        testRegisterForeignToken();
+        address dotToken = MockGateway(address(gateway)).tokenAddressOf(dotTokenID);
+        uint256 fee = IGateway(address(gateway)).quoteRegisterTokenFee();
+        vm.expectRevert(Assets.TokenAlreadyRegistered.selector);
+        IGateway(address(gateway)).registerToken{value: fee}(dotToken);
+    }
 }