I'd never recommend getting into the habit of femming a fem. In a pinch, I'd give it a go, but I'd also think twice about purchasing expensive femmed seeds in the first place. I think the next time, since I'd have to employ femming techniques anyways, I'd chose stock from a seed bank that offers quality 'un-femmed' genetics, and fem 'em myself. Otherwise, you limit your options. My problem is, I'd run-out of the femmed seeds in no time, then what...fem a femmed fem?

You could also pollinate a plant from a femmed seed with pollen from a different strain, but that defeats the reason you purchased the good genetics in the first place.

Possible, yes.
Likely they'll be fine females...yes.
Guranteed to be fine...no.

Genetics degrade if in-breeding is employed as a consistent breeding strategy.