“Accountants do it with double-entry.”
This dirty joke is a double-entendre. It implies a sexual situation, but really it refers to the fact that bookkeeping is a double entry system. You enter information in a total column and a transactional column (hence, two entries.) The informant is an accountant and I believe that one of the major reasons the informant likes the joke and repeats it is that his profession is generally viewed as boring and accountants tend to be stereotyped as suit-wearing, brief-case carrying, straight-laced professionals who spend their days working with numbers and doing taxes. The dirtiness of the joke is unexpected and telling the joke seems to be a way for the informant to prove that he is more than a drone.