A double dump is when one takes a shit, wipes then flushes and whilest washing their hands gets a clogged up feeling in ones anus and then realises they have not yet finished taking a shit, and has to get back on the bog and finish the job.
Two girl's standing by sink in public toilet.
One girl says to the other...
"yo girl i just did a double dump"
"OOoo that nasty! i hope you washed your shity hands" replies the girl.