The opposite of a half sitter.
In a half-sitter, one poops without having made full contact with the toilet, whereas in a double sitter, one has already wiped and washed, and after concluding the poop must return very soon after.
A: “Damn bro, why did you walk out of the bathroom, stop, and immediately turn around and go back?”
B: “My poop turned into a double sitter”
When a dog takes a dump and another dog comes along and takes a dump right on top of it.
Oh no, I stepped in a double dog dump.
(While a double-dog-dump is extremely rare in nature, metaphorically it is quite common in humans; so that the second person can put all the blame for a bad deed on the first person.