When using Content Relations (multiple) field type, it is possible to select the same Content multiple times. See screenshot for UDW:

Once, UDW is closed, you can see that the same object are actually listed multiple times:

In 2.5, it was not possible to select the same Content in UDW.
This is how it looks before selecting anything:

Afterwards, the other listings of the same objects are not selectable:

In the example above, you are not able to select two locations for the same object.

Also in 2.5, the UDW will know which content are already selected for the Content relations when it opens:

1. For the Content relations, open UDW
2. Select "Folder with multiple locations"
3. Close the UDW
4. "Folder with multiple locations" is listed in the Object relations list
5. Now add more related objects to the Content relations
6. You'll not be able to select the same or other locations of the content you selected in step #2

Once you save draft/publish the content containing the Content Releations, the related object is only saved as related object once.

So the problem is purly cosmetics/bad user experience. Data is stored correctly in database