In case of doubt: enter
If a thought comes to my mind, I write it down and process it. During processing, I try not to spend time to see if this was already entered in my system or not.
This can lead to duplicate entries, but you should be able to spot those when going through your lists and certainly not later than your weekly review.
I have experienced that double entries can indicate that there is something missing and that the NA I have captured might not be the real NA.
Example: I jot down "call Dave re: budget 2014"; during processing, I put the NA on my @ calls list.
If the thought occurs to me again that I need to call Dave about the budget, it can mean two things: it is really important that I get this done and I better set up a meeting/conference call to block sufficient time in both our schedules or I have to call Dave about the budget, but first I need to check how we are doing this year against budget so we can have a proper discussion.
In both cases, the NA I had on my list was not the proper NA.