From a functional standpoint...

Frog for double ally protect which if built correctly (preferably with stuff like Regen, Immortal, etc.)

then Ruella due to the turn meter manipulation and an overall great kit. 

Ok!

From Shadowkin, which combination will be better? I have Chani (50), x2 Hotatsu (40), Fenshi, x2 Umetogi (40), and x2 Frogs 50 and 4* (10). 

How far along are you in the game, if you don't mind me asking?

I'd want at least one frog 60'd for general content (usually Dragon + CB, tremendous champion in CB). The others minus Hotatsu aren't necessarily great options to bring up outside of FW. I'm more hesistant bringing up champs specifically for FW until we're later on in the game, for what it's worth.

As is I think I'd take Chani, a Hotatsu, Fenshi and 2 Toragis. 

Sorry for long response.

I`m on 66lvl. FW - 528/945. Next Shadowkin will try your solution. And now i got second Miscreated Monster.

Player level is completely irrelevant. When people ask how far along the game you are, what they are referring to is what milestones you've hit - eg, can you auto all four level 20 dungeons, are you getting UNM / NM top chests, are you clearing DT/DTH, etc.

Ok! 3 key NM. Auto: Dragon Hard 5, Spider 25, Ice 21, FK 20 (normal). Cleared normal DT, hard only 4 rooms.

But my bigest question is - do i need to bring duplicates in team fighting faction wars?

The short answer to that question is - if they're useful, why not?

You don't need to per se, but if they help you get a team set up that can 3 star 21 then it's a worthwhile endeavor.

I will say that I would (likely) hold off on building out dupes like that until I had a solid overall roster at the very least, and likely if I were in a spot to start pushing to get Lydia. 

ok. Which from my dupes will be ok, to bring them to FW (level up too).

Frog is the only one that really stands out to me (and maybe Fahrakin)

But again I kind of round back to really only wanting to do that if we're close to Lydia. The big reason for that is if we've got other champs that are worth bringing up now, it's going to be better than bringing up dupes that will be on the sideline until we get near Lydia. Even after that, those champs might be sitting in the vault after we're done.