Electronegativity is the capability of atoms to attract electrons.
Some elements can have the same electronegativity because their atoms can have the same capability to attract electrons. This happens when two atoms share electrons in the same way, they will have the same electronegativity.
It depends on the whole atomic structure, the relations between the nucleus and the electrons, and sometimes this relations can be the same, no matter if the electrons and the protons are not the same.
In conclusion, electronegativity is not exclusive from each elements since some of them share the same value of this capability.