Add duplicate import test

tests/014_duplicate_import.ts

@@ -0,0 +1,9 @@
+// with all the imports of the same module, the module should only be
+// instantiated once
+import "./subdir/auto_print_hello.ts";
+
+import "./subdir/auto_print_hello.ts";
+
+(async () => {
+  await import("./subdir/auto_print_hello.ts");
+})();

tests/014_duplicate_import.ts.out

@@ -0,0 +1 @@
+hello!

tests/subdir/auto_print_hello.ts

@@ -0,0 +1,2 @@
+console.log("hello!");
+export = {};