Remove Dups in Excel from combined list-From just 1 of those list

I put 2 list together, seperately. Meaning I stacked them to keep the list seperate. The first 1-500 is 1 list, the 2nd list is 501-1000. When I find the dups, having them highlight, it seems all mixed. Some from the top, some from the bottom. How can I tell it to pick the Dups from either the top or the bottom so I know which list they came from?