I run into this all the time. I recollect it and I end up with duplicates in my capture system. It is usually no big deal, the duplicates work out in my weekly review (when I do one). If it happens a lot for a particular item it could be a warning sign that I do not have a clear enough desired outcome for the idea or is the idea even a project?
does it belong in someday maybe? or maybe I haven't come up with a real next action for the project; Is it sitting on my next action list as a mini project? Or is it in the wrong context? Is it listed as @computer when the real NA should be @calls?
Ask yourself these questions if you see something crawling back into your head a lot and you will eventually beat the habit.
Remember this system (GTD) takes conscious practice (Verb) in order to make it a practice (noun).