I think he is a very cute fish. Maybe he had some kind of damage that caused it to fuse together or maybe he really wasn't a DT. Who knows? He is still really pretty and his tail is so long.
Oh I did a search and I guess he would be called a heart tail. That is when a double tail is fused partly and makes a heart shape. Or it could even be a fuse tail. lol Very interesting. Here is the link: http://www.bettaterritory.nl/BT-AABtailtypegenetics.htm
go to the very bottom and it explains it. :) I hope this helps.
ANother link: http://watershed3.tripod.com/types.html
"Hearttail- Hearttail Bettas are actually a type of Double-tail.
They are Bettas with tails that are right in between true full Double-tails, and Fuse tails. The split of the double-tail is incomplete to the point where all there is a tiny split, like a notch in the tail to make it look much like a heart, hence the name Hearttail!! This is a rare "strain" or should I say (Double-tail) variation. Even though it is just a bad Double-tail, it is highly sought after and many people try to develop this but end up with true complete Double-tails." (http://members.fortunecity.com/markang/bettatypes.htm)
I guess it is pretty rare.