Why is Dark Kiss performed on every post- Sep Tepy Asetian?

Aren't they vampires for all their incarnations and between times?

The Dark Kiss, or Khenmet, is the most sacred Initiation to Asetians, and is the ritual which transforms their Ba into that of an immortal. Each and every Asetian receives the Dark Kiss, with the exception of the three Primordials who were born directly from Aset in the Duat. 
Khenmet transforms them forever and after that point is when they are Asetian, in every future incarnation. 
The Elders, who lived in Sep Tepy, received a slightly different version of Khenmet, directly from the Primordials.

So if they are vampires forever, why is the Dark Kiss received many times?

It isn't. It is only received once.

Then what does the following part of 'Elders' section from Asetian Bible mean:
'During the current incarnation, the Elders and other **ancient** Asetians are not transformed with a Dark Kiss from another vampire. They are already vampires at birth, and that condition reflects on everything they are and feel. The transformation that is done on the energy body of **awakening Asetian** can theoretically be done on any human, even nomwadays.'

Becouse it is written that only ancient Asetians, jncluding Elders don't receive Dark Kiss I've assumed that all other Asetians do. Also the last sentece seems to be indicating so. But I have no idea why would that be. Could you point me to some other part of AB or any other work in which there is some explanation?