Serana always hits Frea and they end up fighting and i have to restart from my last save ! wtf! ive used CALM spell but nothing happens.

Had Serana accidentally hit my summons in the past, but that was easy to fix anyway...

Have you tried to fast-travel somewhere, or enter a tavern, if you're in a settlement? Changing area might be able to do the trick...

Maybe send one of them home?

Just let them get beaten (assume they are essential, if not just beat up Serana as she is essential)

Once she get on her knee and the fight will stop

Not surprisingly this happened with serana on my last save except she kept fighting the evil horse. Now shes fighting frea while im doing the main mission for DB and i cant send any of them home because they are mission followers (picked up serana but havent taken her to the castle)

Use the strongest calm spell you have. Dual cast it. It SHOULD work (it does for me, I use Pacify). Then have one of them wait there, and fast travel away. Use the Wait function to stay somewhere for 3 to 7 days, then come back to where they are. They might have gone back to their homebase.
EDIT; If you're in the boat of the guy above me, it MIGHT work to just fast travel with both of them. Usually it does. If not, you're gonna have to find a way around it.

got it, just fast traveled about 6 times then it stopped, it didnt help that i had mercurio too lol.

If you are on pc, try the following command:

actor setrelationshiprank target 3

where actor is one of your followers and target the other. You'll have to do it twice, one for each. This will set up their relationship to "ally" and I presume if one of them accenditally hits the other, they won't fight, like when you hit them accidentally.