You can register up to two emojis to send as reactions. For example: 😀😀
However, there appears to be a bug where you can register an unlimited number of certain emojis.
For example: 🙂↕️🙂↕️🙂↕️🙂↕️🙂↕️🙂↕️🙂↕️🙂↕️🙂↕️🙂↕️🙂↕️🙂↕️
My guess is that the latter are emojis that can be represented by combining two emojis into one.
🙂↕️ = 🙂 + ↕️
🍋🟩 = 🍋 + 🟩