There are no duplicates in a MAME romset, there are version variations and regional versions, but no duplicates.
You specify the region you prefer in the import wizard, that version will be the default launched if that game has a rom from that region. Also what is "correct" in your example of Pacman, none of those roms are technically correct, as the actual parent rom is puckman, not pacman, so in your example you would specifically be prioritizing the INCORRECT rom.