>>468
You can use こと multiple times in a sentence without sounding foreign. Actually your first example sentence is perfectly grammatical and sounds natural. Technically you can use こと more than twice if necessary, e.g.,
彼女のことを聞いたことがないなんて、おかしなことだ。(It's strange that [you/they/whatever] have never heard of her.)
As for the subject and particle problem, there are several ways to circumvent the no-multiple-は rule. A quick fix would be using another particle that is similar in meaning. In your case, を does the job:
彼らは彼女のことを聞いたことがない (Not important, but 彼ら sounds more natural than 彼たち.)
Replacing 彼女のことを with 彼女について also works fine in this case.
Another sophisticated way is changing your wording entirely. This requires a firm grasp of Japanese grammar and large vocabulary. Also, this may fall into the category of style, so I guess you don't need to systematically learn how to change a sentence.
That said, using は/が twice is ok in an informal situation especially when some artificial fix is needed to avoid double は/が. Certainly it is awkward if you "write" 彼らは彼女のことは聞いたことがない and it's grammatically wrong, but native speakers often "say" that kind of sentence in conversation and the sentence makes perfect sense.