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It has to have a new invitation letter each time, so accurately speaking, this can't be treated as a different supporting document. I think better to show visa officer your parents' close ties to China, like real estate(picture, certificate, etc), other children(picture of whole family, notarized relationship certificate) in China and maybe more money this time.
You may also write to visa officer(I tried it after my mother in law was refused twice) apart from the invitation letter to your parents. Good luck!