Should I keep duplicates?

I'm wicked new, only about 3 weeks or so in.

So far I've gotten a fair number of duplicate Uncommon heroes and a few rare duplicates. Should I be keeping them generally? I went to use an uncommon to level up some food and it said "you shouldn't use higher rarity heroes" or something like that... but I assumed that was just a general warning in case you accidentally put someone in by mistake...

As an example for uncommon, I have 5 Spiritwalkers in my reserve vault... google says "decent, niche support champion for early-game players"... I don't need 6 of her do I?

For Rare, someone like Sorcerer from undead hordes. Google says "considered a mediocre, early-game Rare champion"... i have like 3. Am i good to just feed her to someone else?