jilbaab

Q: Is it necessary to wear a jilbab (a garment that starts from the top of the head), or is it permissible to wear an abaya (with the same coverage but starts from shoulders) along with a hijab?

Q: If a woman is wearing the jilbab and a niqab, does she also have to wear a khimar over/underneath the jilbab though the jilbab covers her head anyway? I heard some people say that she has to wear both the khimar and the jilbab, is this true?