Phyrexian Infiltrator

• WWizards of the Coast2004-10-04
  Official Ruling
  There is no effect if the same player controls both creatures when it resolves.
• WWizards of the Coast2004-10-04
  Official Ruling
  It is possible to activate this ability in response to itself and generate some odd combinations. For example, if you control this card and another creature, you can use this card’s ability and target the creature you control. You can then use this card’s ability again and target a creature your opponent controls. The second usage resolves first and you get your opponent’s creature in exchange for this one. The first usage then resolves and swaps your other creature for the Infiltrator so you get it back. The net effect is that you can swap any creature you have for any of theirs if you can pay this ability twice. Note that your opponent does get the chance to use the Infiltrator if they have the mana in between your two usages and can mess you up.

Rulings via Scryfall