What I think... There are some combinations that grant BOTH moon and sun... Those are some hybrids with cold and lighting in them... However, there are also designated combos for the moon OR sun, not both. Why do I say this? Explain how Lichen+Crystal landed me 10 suns and no moon? I also heard it's confirmed time does not matter... But if you get a second sun or moon, just display it and try again =)! Luckily, I got both of them before I hit level 20.
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