there is no functionality for merge.
I thing that as a workaround can be to create Picker custom field and assign it to app Person.
With this you can configure it the way that picker will show the people and you can establish relation between those
objects. So there will be way how to link them (and their relations)together.
Its not optimal, but at least its connecting them together.
btw back to the root, they are duplicated, because they have diff emails, or names, in both systems?