GMAT Verbal Section | Re: pronoun in modifier

I'm not even totally sure that's right! For instance, this sentence is probably grammatical, according to the GMAT:

'The book,which had a portrait of George Washington onits cover, was very dull.'

Two pronouns, same antecedent, one is inside of a modifier introduced by the other.

For what it's worth, I've also never heard that cited as a grammar rule, and I've never noticed it being tested on the GMAT. There's a somewhat complicated syntactic reason why the original example is wrong, but