From what you described I think I can see two possible reasons on why things creeping back in:
1. You possibly only captured part of the action/idea you were originally thinking of. So while you captured part of the idea, you subconciously still think about it because you don't feel you captured the entire idea. I do this quite a bit on larger ideas as I'm trying them to form a project. Bits and pieces are captured, but then remelded into an action or project as part of my weekly review.
2. Are you doing you're weekly review on a regular basis and really scanning all your lists? I had this problem when I wasn't doing my reviews on a regular basis and with some depth to them. I'd finally do a really good review and realize I had paraphrased the same idea/action 3-4 times because I wasn't in touch with what was on my list.