It doesn't help. Here's why: the md5_hex of a given value will always be the same. So, if md5_hex("hey") is always the same, then md5_hex(md5_hex("hey")), while it will be a different digest than the first, will be consistently the same. Try it yourself.

If the value for the first round of md5_hex isn't random, no amount of repetition will create a unique value. If you were using an encryption rather than a cryptographic digest algorithm, then the extra pass may help (depending on the algo.).

HTH. (BTW: I'm not a crypto expert either, but I have done some amount of research trying to better understand it. If I'm Full O' Shite™, please tell me!)