My tenants are living after 4 months, they signed a lease for 6 months. Should I give them the deposit back? should they pay for the other 2 months?

They signed a contract with you for 6 months - they owe you 6 months of rent. You have an obligation in "most" States (where are you?) to try to rent the apartment and collect the 2 months (after they leave) from a new tenant.

You can collect rent twice for the same apartment but if you can't rent it out you can expect them to pay you.

That lease is a contract and you can take them to Court to enforce it - or they can take the refund of the deposit to Court and contract law will prevail.