It's not allowed to have more than one, but that's more because it simply makes no sense for one person to have more than one. As far as "crimes" go, it's not really up there, and therefore they allow you to merge the duplicates.

There really is no real reason to have more than one (and I admit it is a bit strange that the OP had two, with different names. It could have been a bit complex as they would presumably have been required to provide some kind of evidence that they were both held by the same person - and I'm not aware of many people having valid ID showing two different names [though I am one such person!]) so it's not like holding two different accounts is a crime that FB deems punishable.

I would guess that the prevailing assumption would be that two accounts had been mistakenly opened due to mere oversight.