Is it permissible for a woman to perform Hajj a second time without her mahram (her husband)?

A wife's doubt about the validity of her Hajj does not invalidate it unless she ascertains the existence of something that would invalidate it, because the default is the validity of the act of worship. What she intends to do is a voluntary Hajj, and it is not permissible for a woman to undertake a non-obligatory journey except with a husband or a mahram. If something that invalidates Hajj has occurred from her, it must be clarified.