Will the PhD degree be retracted if the thesis is found to be not novel?
It would be very rare for such a thing to happen. However, it might depend on the circumstances behind it. If it was nothing more than a misjudgment by the candidate and the committee or some missed (but relatively obscure) information that wasn't included, probably nothing would happen. People would just say "oh well..." and let it go. 

However, if some misconduct occurred, such as hiding information or plagiarizing other work, then it could result in withdrawal. But then, it wouldn't be for lack of novelty, but for the misconduct. 

Novelty is a judgement call in any case. Looked at in hindsight something may appear to be not-novel when, at the time, it was. Even at the same moment, different people might judge it differently. 

If you are the candidate in question, I recommend that you rest easy. Likewise if you were the advisor.