Yeah, this is a pretty common glitch, it will usually happen if you click the post button twice by accident, you might even click it 3 times if your internet is bad, or you're just really impatient lol. It's a very annoying glitch that bugs almost everyone, but you do end up getting double TBT bells, so not all is lost.
Also, I should mention that this should probably have been posted in the "Bell Tree HQ" subforum. This subforum is for New Leaf threads!