The Doubles Operator selects all vertices which are recognized as duplicates.
Note: this operator does not preserve the original vertex selection. So after the operator is called, only the duplicate vertices are selected or no vertex is selected when the mesh is clean (has no duplicates)
How to fix:
- Inspect the mesh to check that it is OK to merge
the selected vertices with their doubles.
- Press the repair icon right to the Doubles Operator (see image)