Well, I think that all hard links will "take on" the creation/modification date of the original file... so, if you have three hard links to one file, ALL the hard links will have identical creation/modification dates.
I think I have another suggestion -- 'ls -i' will give you the inode number for each file. You could then compare the inode numbers to make sure you don't "dupe" a file along the way.
Argh... at this point, I'm kinda grasping for ideas. I do believe that what you're trying to do is inherently "intensive" since you are doing a lot of "looking" at a lot of different files.
If I think of something magical, I'll post back, but I think we're bearing down on the conclusion that there are a number of ways to accomplish this, none of which will be lightning-quick, unfortunately.
