Why did Cain and Seth have three descendants with the exact same name?

We do find several descendants of Cain with the same name as those of Seth, subject to translation of the original spelling:
Adam ............................................. Adam
........................................................ Seth
........................................................ Enos
Cain ............................................... Cainan (Cain)
Enoch ............................................ Mahalaleel (Mehujael)
Irad ................................................ Jared (Irad)
Mehujael ....................................... Enoch (Enoch)
Methusael ..................................... Metheluselah (Methusael)
Lamech ......................................... Lamech (Lamech)

This would be more apparent in the above comparison if the order of Mahalaleel, Jared and Enoch are reversed. The reason is no doubt the syncretism of two originally separate traditions.