Yes, each value could be represented in a select field multiple times, but the values need to be present in the choices multiple times in order to select them multiple times. The fact is that whether this is a select2 field that uses a bit of ajax or if it was a regular select field, the values are only present in the field one time and they are marked as selected or not selected.
In the case of the post object field, that uses select2, you would need to build additional JavaScript the duplicates any value selected so that it is present a second time so that it could be selected again.
I looked over the first few results and it does not appear that this is possible, even with a select2 field, but this is where you’d need to look.
https://www.google.com/search?q=select2+select+the+same+value+multiple+times&ie=utf-8&oe=utf-8
ACF currently uses select2 version 3.5.2
