Not sure if this is what you are looking for.
You can always get the last signature of the client and show it next to the new signature in a page or pop-up, so you can compare it with the naked eye and accept it.
Because there are 2 different signatures it is not possible to compare the Base64 string
Maybe you can try an Image Similarity API (Image Similarity API | DeepAI). or (GitHub - htmlcsstoimage/image-compare-api: Simple API for comparing images)
Just some thoughts :)
comparing two pictures of different recordings of a signature is beyond the standard capabilities of Mendix.
I recommend to check if there are any existing services for this kind of task that offer a proper REST or webservice interface. Then you could just integrate with that service.
I haven’t heard of such a service yet though – it sounds like quite some complex image processing problem.