Additional tests for using for directive

test/libsolidity/syntaxTests/operators/custom/duplicated_operator_binding.sol

@@ -0,0 +1,13 @@
+type Int is int32;
+
+using {add as +, add as +} for Int;
+
+function add(Int, Int) pure returns(Int) {
+    return Int.wrap(0);
+}
+
+function f() pure {
+    Int.wrap(0) + Int.wrap(0);
+}
+
+// ----

test/libsolidity/syntaxTests/operators/custom/duplicated_using_directive.sol

@@ -0,0 +1,14 @@
+type Int is int32;
+
+using {add as +} for Int;
+using {add as +} for Int;
+
+function add(Int, Int) pure returns(Int) {
+    return Int.wrap(0);
+}
+
+function f() pure {
+    Int.wrap(0) + Int.wrap(0);
+}
+
+// ----