An accountant knocks on the door of his doctor's surgery and walks in.
"Hello, doctor. Please help. I just don't know what's wrong with me. Goodbye."
With that he turns around and walks out.
30 seconds later he is back. "Hello again, doctor. Please help. I just don't know what's wrong with me."
The Doctor stands up behind his desk, looks the accoutant straight in the eye and says:
"Mmm. I think you have a serious case of double entry."