>>1769269
Dupes give basic hp and attack stat buffs, but the stars after every 5 phases unlock upgrades on passive skills. 3 star units also unlock sigils. Not sure what those are but probably more number buffs.

They're important but also not really if it makes sense. It's mostly whale incentivization to drop money but we'll see how generous the events are for giving XXL shards to star your units.

Tl:dr nothing to worry about unless it's your favorite unit, then pull dupes to max out their stats