Team Constructed tournaments use Unified Deck Construction rules: Except for cards with the basic supertype, no two decks on a team may contain the same card, based on its English card title. (For example, if one player is using Naturalize in a Team Constructed tournament, no other player on that team may use Naturalize in their deck.) No players may use cards that are banned in a particular format. Cards that override deck construction rules (i.e. Relentless Rats, Shadowborn Apostle, etc) may only override them for one deck on a team.
Unified Deck Construction rules are only applied when all members of a team have decks of the same format.
Note the change here! Prior to this update we had the “four of” rule, that simply applied across the team instead of across a deck. Now we only allow one player on a team to use any copies of a card. This makes checking legality considerably easier, both for teams and judges.