I don’t think anyone answered your question, and I’m not knowledgeable to answer. However, when I have grammar questions that I can’t quickly answer, I’ll throw the sentence in Google translate to see how Google structures the sentence. Grammar is applying rules, which a machine is excellent at. Tossing in your sentence, Google does NOT add the second ‘fe’. Looks like your sentence is correct, Tatjana.
…nid oedd yn siarad Cymraeg o gwbl