allow self payments

I'm guessing this was disallowed as a sanity check, under the assumption that no one would ever want to do a transfer to and from the same account.

While this wouldn't be a common occurrence, I think it is possible that in weird cases, a self payment makes sense if you consider that double entry may be part of a larger system. A contrived example would be banking a cheque made out to your self. The cheque clearing system may decide that the cheque is valid, and process the transfer from your account, to your account, as the bank wants the 25c fee. The transaction would need to happen so it appeared on the customer's statement, and while the end result is no change, for all the records to match up the transfer would need to take place.

I don't see any particular danger in not prohibiting self payments.