The funda stand good.
If the Mass is same then velocity will be lost after collision.
If mass is different then velocity will not be lost completely. May be the body of higher mass will continue to follw the path will reduced velocity after collision. Then the velocity reduced will be square of the mass* force exerted by the lower mass body during Impact.
Secondly the higher mass body may change its path and this change of path will depend upon the nagle where these particals collide and mass difference of the particals.
Buth in both the cases velocity will be lost and it will not be c2 or 2c.