When you Cleveland steam a girl then break up with her immediately after
Chad: I double dumped Lisa last night.
Mike: that must have been a messy one
When you drop yo load inside your girl twice in one night.
Guy:Hey want to fuck tonight?
Girl:Sure how about a double dump?
Guy:We gon' need plan B tomorrow
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.