I honestly didn't expect to win the last one, so missed out on picking this weeks topic (would have had Network Southeast but this one will probably get more pictures, its a good choice though (I'm sure i'll find something foggy!)
There did used to be something in the competition guidelines about it, but it has been removed. I believe you can enter any photo multiple times, provided it hasn't previously won or isn't anywhere on your profile (avatar, profile picture, sig etc)
I have entered a few images twice, although I try and find new ones where possible
Just spent two hours going through every possible pic, and I'm stumped, at best I have something tenuous which I don't want to enter because it's such a poor pic, so unfortunately no entry from me this week.