All demons have a healing ability (Shaco Theory)
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Swain (Raum) has ult and passive.
Evelynn has her passive
Tahm Kench has his Q and E
Nocturne has his passive
Fiddlesticks has W
Shacos healing could honestly be his R. Think about it, maybe he overheals himself so much to the point it destabilizes his form and creates a duplicate. It fits with the fact that he is the demon of chaos.