If it's an exact duplicate, then there should be a 50-50 chance of winning/losing, so half the party should die. Except...
1) If it's the whole party vs one-at-a-time duplicate, the party should win most of the time
2) PCs know their characters way better than the DM will, even if the DM has the same character sheet. The PCs will be able to remember details the DM will miss. Except that since you're on this minmax board, you might be able to pull tricks the PCs would never have thought of.
3) The duplicates aren't worried about using up expendables. They don't have to save anything for later.
Overall, I don't think it'll be a TPK, but I would expect some PC death.
I would assume the duplicates are of the characters as they are at the moment. So if the PC has 25hp damage, the duplicate will also have 25hp damage. If the PC has a Bull's Strength cast on him, the duplicate will have a Bull's Strength. One thing that might be problematic are effects that affect "allies". If it affects the ally when cast and then runs for the duration, then the duplicate gets it. If it's something that you have to be an ally round-to-round, then the duplicate won't get it.
On the fun side, I could easily see me using it as a joke item. It functions "4 times a day". PC looks in the mirror. Duplicate appears. Duplicate is also looking in the mirror. Duplicate of duplicate appears. Duplicate#2 is also looking in the mirror. Duplicate#3 appears. ... Duplicate#4 appears. Duplicates fight, wipe each other out, PC just stands there and watches the entire time.
-or- Duplicate appears. Immediately realizes it's a duplicate and that its "lifetime" only last until the PC or duplicate is defeated or destroyed. So to preserve its own life, it sets about body guarding the PC.
Good luck!