I started a new thread on this topic, precisely because I was hoping a Blizzard dev would confirm my understanding for the record:
Community Discussion > Confirm how the Mulligan works?
That said, nobody has ever been able to produce an actual video or HSReplay showing this allegedly common occurrence. Every time we can check, it turns out to be either: a) The player mulliganed one copy of a card and got the other copy of that card as a replacement; b) The player mulliganed a card and drew it as their first-turn draw after the mulligan (i.e. after the mulliganed cards are shuffled in to the deck); or c) The player mulliganed a card they didn’t like, got another card they didn’t like, and vaguely remembered getting back the same junk they dropped.
If you are the exception, I would be extremely interested to see the evidence! I don’t suppose you have a game replay you can share?