In Pokémon GO, Special Trades can only be done once per day to limit the number of times a single Trainer can make Special Trades with all of their friends. However, according to Niantic, “Special Trades can only occur once a day, unless you’re at certain special events.” Special Trading does become more accessible over time. As the Friendship level between two Trainers increases, the amount of Stardust needed to complete a Special Trade will significantly decrease.
Source: Official Pokémon GO blog
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