On the same note, i do this with locking my car. Regularly i will lock it, walk 15 seconds down the road, think "shit...have i locked my car?", walk back to the car, unlock then lock the car again!
Even when I remember that I have locked the doors (car doors or house doors) I have to go back and check. Very odd.
There have been times when I've been in bed and my thought process has been along the lines of
"Did I lock the back door?...Yes, I definitely remember doing that.....BOLLOCKS!" and I have to go check, even though I know I've locked it.
Thank God for that. I thought it was just me!