without seeing your roster: could see wanting one for hydra, one for doom tower, one of those types of situations but you can probably get by just fine building out one and using the other in faction guardians. 

Im level 18 i have like 30 champions most of then one or two star

in that case i definitely would only build the one. keep the other though, for faction guardians at the very least. :)

Keep the dupes for faction guardians. I can virtually guarantee you'll never build the second one out, though.

Stupid? Whats faction 

When you open up Raid initially, go down to the bottom right of the landing and you'll find Guardian Ring. 

i

After that, on the left side click the "Faction Guardians" tab. You'll then see images for all 14 factions in the game, and since your duplicate epic is in Knights Revenant, you'll scroll down to them. 

From there, you'll scroll down to the rarity that applies to your duplicate(s), in this case epic: 

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As long as those champions are in those slots, your champions of that rarity in that faction will have a permanent stat bonus, listed below. Hopefully that helps. 

Here is the initial post about the Guardian Ring in November in case you'd like any more details: https://plarium.com/forum/en/raid-shadow-legends/843_news/31077_patch-notes--24-11-21--update-4-90/