It is not a stupid question at all, and is often a very real problem. What you are trying to sort out are the well known, "duplicate files" problem.
There is no easy answer to this. You may want to read a new and ongoing discussion about it raised in Lee Koo's newsletter here; How can I get rid of all my duplicate files in multiple hard drives?.
It may not be exactly the same as your problem, but something there may help.
Specifically though, if you use Explorer in Vista, (eg My Computer, My Documents, etc), you can choose what columns to display in the right hand pane of Vista, as in the example here;
That right hand pane shows date, but within the folder itself you can choose whether to display "Created date" or "Modified date", and so on for each of the files.
The problem is that this doesn't really help you find and decide whether two files are identical. There's only one sure way, to look through each and every photo file, and keep the ones you want and delete those you do not.
Sorry, but I hope that helps.
I have been copying my picture files to disks for several years, but I don't know how to just copy the ones not already on disks. I have Windows Vista and can't find a place to enter a start date or something like that. I like to put the most recent ones on disk every six months or so. They of course are in several different files. Thanks so much for any help. I feel kind of stupid asking this, but I really do need the help.