Tournament rule 703.4.c.4.a Example
Example: A player has two total copies of Traveling Merchant on their registered decklist, one in their Main Deck and one in their Sideboard. During game one of a match a player draws and plays one Traveling Merchant. On a later turn, they draw the second copy and realize they forgot to de-sideboard. At this point, since both Traveling Merchants have been drawn, restoring the deck is not possible as there is no easy 1 to 1 swap that can be made to restore the correct deck state.