Jeanne Calment's birth date was definitely verified. There is no real doubt that she was born in 1875 as claimed. She had a daughter, Yvonne, in 1898. Yvonne supposedly died in 1934. There are people who believe that it was actually Jeanne who died in 1934, with Yvonne taking over her mother's identity so she could avoid paying taxes on Jeanne's estate (Jeanne wasn't ultra rich but she had nontrivial holdings by that time), or something of that sort. There is a section about this in Jeanne's wikipedia article. The main article supporting the claim is this:

https://pmc.ncbi.nlm.nih.gov/articles/PMC6424156

The Wikipedia article says why other people think it is wrong. Shrug.