I have recently bought a tank and have filled it with different livebearers. A couple of which are mollies. I originally had 1 male and 1 female however I now have two males. Does anyone know anything about this kind of thing?
The most common reason for this "occurence" is that the fish were not fully grown yet. Males don't get their gonopodiums until a while after they are born, some taking a very long time to get them. So, they look like they have the fan-shaped fins which are associated with females. They then develop their gonopodiums, which makes it appear that they are changing sex.
What size tank is it? Be careful of aggression problems between the two males.
You're not the only one with this problem. Last night I moved a juvenile female guppy from the hospital tank to the main tank as her tail seemed quite regrown. This morning I found HIM with his nose up his sister's bottom and a very definite gonopodium under his own. I suppose I didn't look carefully last night, because I was so sure he was a she, she was last time I looked He even had what looked like a gravid spot (common sense now tells me that this cannot have been the case). Bother, and there I was trying to run my tanks on strictly convent principles!
He even had what looked like a gravid spot (common sense now tells me that this cannot have been the case). Bother, and there I was trying to run my tanks on strictly convent principles! 
