To my knowledge, if a person's eligibility for a HKPIC has been verified by virtue of No. 4 of Part 2 in Form ROP145, the person is ineligible to apply for a HKSAR Passport. But if this person is actually of Chinese descent, and wants to have a HKSAR Passport, then he/she has to prove it, and renounce the foreign nationality he/she possesses. The reason is "Article 3: The Peopleâ€™s Republic of China does not recognise dual nationality for any Chinese national."
Having said that, holders of HKSAR Passport, they can immigrate and have a foreign passport or foreign passports. In this case, if they don't make a declaration of renouncing Chinese nationality, their foreign passport(s) are just deemed travel documents. (See explanation 4: Chinese nationals of the Hong Kong Special Administrative Region with right of abode in foreign countries may, for the purpose of travelling to other countries and territories, use the relevant documents issued by the foreign governments. However, they will not be entitled to consular protection in the Hong Kong Special Administrative Region and other parts of the Peopleâ€™s Republic of China on account of their holding the above mentioned documents.)