The dealer claims that the pill is "double stacked", ie contains twice the amount of MDMA and sells it for a higher price.
It's a lie used to shift more pills and pull in more profit.
Double stacked pills often are slightly larger, because they contain twice the amount of filler.
A.) It has become clogged or
B.) You're having a Double Stacking contest
DOOKIE: NO! What'd you do man?
J: Well I tried to flush it, but it just spun around a bit. There was no way I was gonna make it home without squirting all over myself.
DOOKIE: Dude, tell me you didn't....
J: Yep, I just Double Stacked it.