Are all legendary Pokemon the same CP?
Yes, all legendary pokemon of the same species have the same CP value.
Source
All raid bosses have the same CP value for any given Pokémon species, because they all have the same IV values and level. This is true whether you are talking Legendary or not.
The Pokémon itself (the one you catch) will not have the identical CP values; they range like the Pokémon you catch from eggs (10/10/10 to 15/15/15, level 20).