In a best-of-3 match, if both players pick the same battlefield, does one of them have to swap it?

Ruling: No, if two opponents flip up the same battlefield, play continues normally. There is no rule requiring a player to change their battlefield once it has been revealed.

Nuances:

• A player can only use one copy of a battlefield in their deck
• In team formats, a team as a whole can only use one copy of a battlefield between all team members
• This applies even in mirror matches where both players might have the same battlefield as their only option