If by INVENTOR you mean APPLICANT... It is simply first to file. If you filed a patent for someone else's work, that work has to be successfully applied as prio

If by INVENTOR you mean APPLICANT... It is simply first to file. If you filed a patent for someone else's work, that work has to be successfully applied as prior art to invalidate the patent. If the existing work isn't sufficiently documented by acceptable publication, it is difficult to use as prior art years later when the patent is first litigated. But, I am not a lawyer.