Re: is running 7 passes twice the same as running 14 passes
or are some of those passes possibly running over old passes?
Each and every pass "runs over" the older passes. Each pass writes data to all the drive's freespace once. So if you do a run with seven passes, freespace is overwritten seven times. Same goes for overwriting a file.
So I believe the answer to your question is "yes".
What's more, for most intents and purposes, one overwrite pass should be plenty--unless you might have certain agencies with three-letter acronyms for names after you, in which case I'd seriously consider physical destruction of the media in lieu of an endless series of overwrites. But that's me...