If I'm not mistaken, BEWITCHED and DARK SHADOWS shared several of the same years together on TV, and both were featured on the same network, too.
Considering how much Dan Curtis enjoyed "borrowing" from other sources, I'd say that the Angelique/Cassandra- Samantha/Serena plot was more than just coincidence.
Of course, Serena dressed in far groovier duds, but then again, she wasn't stuck with Ohrbach's cast-offs, either!
I just read that DS and THAT GIRL were both on TV from 1966-1971. I loved both shows, although they were very different, and never realized that little fact.
Love, Robin