One of the staple features of Pokémon GO is that of trading, but the feature has certain limits and restrictions in-game. Trainers are allowed to send their Pokémon over to a friend in real-time, usually to help them complete their Pokédex or increase the odds of receiving a Lucky Pokémon. After transferring a Pokémon, however, matters can get somewhat more confusing. Attempting to trade a Pokémon more than once can run into the rules set for the feature, as can trading certain types of Pokémon or creatures in particular conditions.

https://screenrant.com/pokemon-go-trade-back-lucky-once-limit/