I don't know exactly how this can happen, but recently many dup txs can be found if you look at steemd.com carefully.

sometimes transfer tx comes with virtual op (on steemd.com UI), which doesn't make sense to me.

API history call returns two txs separately, so unless you have a check routine, dup transfer may occur.

don't know if this is a chain-level problem or api-level problem though.