Assuming the hash is being cracked using brute force techniques, hashing the hash would mean that theoretically you will need more time and resources to crack that hash due to the fact that you need to execute multiple iterations of the algorithm.
However, having said that, if it is time you are looking for MD5 is most certainly not the way to go. In order to make a hash as difficult to crack as possible I'd recommend a "slow algorithm" like PBKDF2.
By doing multiple iterations of PBKDF2 you will make the process of cracking the hash so slow (relative to other algorithms) that it will become virtually impossible to crack the hash in any reasonable amount of time.
Edit: When I say multiple iterations I don't mean 2 or 10, I mean thousands or hundreds of thousands depending on the acceptable amount of time you are willing to wait for a legitimate hash to be generated or compared.