A gimmick used by E dealers when they sell to idiots.
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.
If you buy double stacked pills frequently, get off E and go back to pot. Hard drugs should only be used when you know how they work and what they do.