Hi Jason,
Def MUST delete one. It's a violation to have 2 claimed listings in the same or even different accounts.
WARNING: If the stats are the same that indicates the 2 have merged. But even if stats weren't the same yet, if the listings are very similar they have likely partially merged even if the stats don't reflect that.
So important that when you delete you click option to remove from account NOT delete from maps.
If you delete from maps it will often delete everything since they are merged.
But deleting from acct just does that - takes it out of account so you can't get a penalized. It does not delete it from maps/live.
IF NAP is slightly different, then when that listing is released from dash, sometimes it will break off into a separate live dupe. So watch for it and if it does, then get rid of it by direct edit on the listing and report as dupe.
Warn client not to claim dupes any more too. Sometimes they claim and suspend and that no longer works and again is not advised.
Did that help answer your questions?