Is Danielle really dark danny's daughter?

Technically speaking, she is him, being his clone.

well theres 2 daneills . one Vlad made with cloning and science and one dark dany had with Mandy.(afterbirth) "Dark Danny" is made of both of what was left of Danny phantom when he removed away his morals and humanity with special claws. (friends + family died so he wanted to escape the pain) and also he is Made from 1/2 Vlad.

but its और like danny just absorbed his evil to transform himself. because he looks alot और like an older danny with vlads powers than he does a equal fusion of the 2 chracters also he identifies himself as Danny most of the time. but basicly the 1st. dani was made द्वारा Vlad but with 100% of orginal danny DNA. the सेकंड danielle(afterbirth) is like 25% danny 25% vlad and 50% Mandy.