run addMissingFunctions only once

test/libsolidity/syntaxTests/missing_functions_duplicate_bug.sol

@@ -0,0 +1,28 @@
+pragma solidity ^0.6.0;
+
+pragma experimental ABIEncoderV2;
+
+contract Ownable {
+    address private _owner;
+
+    modifier onlyOwner() {
+        require(msg.sender == _owner, "Ownable: caller is not the owner");
+        _;
+    }
+
+    function renounceOwnership() public onlyOwner { }
+}
+
+library VoteTiming {
+    function init(uint phaseLength) internal pure {
+        require(true, "");
+    }
+}
+
+contract Voting is Ownable {
+    constructor() public {
+        VoteTiming.init(1);
+    }
+}
+// ----
+// Warning: (324-340): Unused function parameter. Remove or comment out the variable name to silence this warning.