Does the prayer of one who unintentionally says "Ya Rabb (O Lord)" after saying "Rabbana wa lakal-hamd (Our Lord, and to You be all praise)" become invalid?

Uttering "Ya Rabb (O Lord)" during prayer does not invalidate it, even if the worshiper does so intentionally, because it is not an extraneous speech to the prayer. If done out of forgetfulness, it is even more unlikely to invalidate it. However, one should not intentionally use this phrase in this context, as it has not been reported to be used in this specific instance.