According to the games: Celebi can't breed (unless you use glitches or hacks). According to the movie: there are multiple Celebis, so breeding or multiplication is possible.
I think when Celebi is ill and does a time-travel or uses too much power for a time-travel, during the travel he unintentionally clones himself by being in two timelines.
Another theory: His past or future self is visiting him at his current time ||=(:3
According to the games: Celebi can't breed (unless you use glitches or hacks). According to the mov
