It is one thing to write a will. It is another to prove that when the will was written the will-maker had the capacity to appreciate the consequences of doing so.
Once a doubt is raised as to the existence of testamentary capacity, an onus rests on the person propounding the Will to satisfy the court that the will-maker had the requisite mental powers. Specifically did the will-maker:
Where a will-maker is not found to have the requisite capacity the will in question will not be able to be probated. However, the prior will can be (presuming capacity at that time). See In the estate of Ruth Annie Howie
Where questions are raised regarding capacity (and more than one will has been made), it is not sufficient to seek probate of and earlier will on the basis that the the will-maker may not have had capacity at the time the later will was made. The correct course of action is to seek probate of both wills in the alternative so that the Court can determine the question of capacity. See In the Estate of Norren Joyce Watson.
Another issue to consider in the context of capacity is the implications of testing and developments in future capacity predictions and what questions laywers might need to be asking of their clients in the future. See Medical tests for capacity.