Fix a case where snippet is specified twice

When importing a file across multiple locations in a module make sure it
doesn't trip an assert and it works as expected.

tests/headless/snippets1.js

@@ -1,3 +1,9 @@
 export function get_two() {
   return 2;
 }
+
+let a = 0;
+export function get_stateful() {
+  a += 1;
+  return a;
+}