In Polish you can do both, and at least to me they feel slightly different - the first "a" represents the small pause while you quickly take in the air before you start laughing. To me this indicates an element of being surprised, and the reaction being natural. I think that "ahaha" is a lot less likely to be used sarcastically than "haha".

Anyway, those are just my intuition as a native speaker - I'm not sure whether it is universal, I never discussed this with anyone before.