If you have the slightest doubt that you don't want to do study medicine, then a double-degree that you do have an interest in could be the start to an alternative career. If you know you do want to do medicine but you change your mind later (life happens), then you can start with a single degree and "transfer" internally within the uni or otherwise into a double-degree so long as you have the marks to do so. It usually doesn't lengthen the total length of your degree.
But keep in mind that doing a double-degree would increase the number of times you can sit the UMAT.