Translation:This is the engineer who is bilingual.
Yes, the second ο is necessary -- the relative pronoun ο οποίος always has a definite article in all its forms.
This also helps disambiguate cases, e.g. Αυτή είναι η γυναίκα η οποία βλέπει. "This is the woman WHO sees." versus Αυτή είναι η γυναίκα την οποία βλέπει. "This is the woman WHOM he/she sees."
The οποία is the same here but you can tell the case of the relative pronoun (nominative versus accusative) by the form of the definite article, η versus την.
I'm not so sure: "who's" is how people say it in English. It doesn't look strange to me and sounds better. The problem is that people often do not know how to spell the contraction and get confused by "whose" in writing, but the contraction doesn't cause confusion when speaking (native speakers).