A portmanteau of 'retard(ed)' and 'redundant'. Used to express that something is both superfluous and stupid.
John 23:04: You're all going to hell. o.o
Jimmy 23:05: Dually noted.
John 23:05: You mean 'duly noted', right?
Jimmy 23:05: No I mean dually, in that I noted it twice.
John 23:05: That's retardunt.