The first part shouldn't be a problem. Hashes are not guaranteed to be unique.

I can't say why you get different values every time you hash the String module. It would seem like you actually create a new instance of String every time you hash it. If you do this:

mixed x = String; hash_value(x);

(3) Result: 8486372

hash_value(x);

(4) Result: 8486372

You get a more expected result. This may actually be a problem, but I can't say for sure.