In accordance with Nuzlocke law, only the first encounter in any area may be legally added to one's team. If you already encountered a Pokémon in the same location, the gift Pokémon is no longer valid, and if accepted, must be relegated to the 'dead' box. Similarly, if you have accepted a gift Pokémon in any route (or city if you allow encounters in both), no further Pokémon may be caught there. In some cases, the only possible encounter is a gift (N's Zorua in Driftveil). More demanding Nuzlockes would outright ban use of gift Pokémon.