I suggest you test with another keyboard. I'd suspect doublestrikes on the
Return Key before I suspected a glich in the program.
Or maybe not.
When I was using a Mac Plus, I spilled some soda on it. I thought I
cleaned the keyboard up properly, but suddenly I was getting two
characters for one key press on one key. So I take the keyboard apart,
desolder the keyswitch and put in a new one (obviously I happned to have a
keyswitch that fit, otherwise I wouldn't have done this), put it back
together and it still did it.
It was a bug in that early Microsoft Word for the Mac. Pressing that one
key (I think it was only one key) and the numeric keypad caused the
character to be entered twice. If I'd pressed any other key after
cleaning the keyboard, I'd have never noticed, or if I'd used some other
program, I'd have never noticed.
Not likely to happen to someone else, but I really thought it was the
keyboard that time (especially since I had spilled the soda, so there was
something to blame).
Michael