Board Thread:Questions and Answers/@comment-211.203.21.6-20150704051606/@comment-26044650-20150704053456

The 6+1 Big Set does have a "guaranteed 3 S rank treasures" attached to it, so you're always going to get at least 3 S rank treasures if you buy the 6+1 Big Set. The regular Supreme Treasure Chest does not have the perk.

Anyway, you're always going to get duplicates when you open treasure chests. Always. It's just the nature of the RNG - just keep at it and you will eventually get the treasures you need!

EDIT: Misread the question and changed answer so it's actually what the OP is looking for. That's what I get for trying to understand something at 1:37 in the morning. :P