In case both upper and lower value handles (for range selection) have same value, it becomes very difficult to operate slider. You have to pull left handle to the left, but there is no visual clue that you have to do that.
Observed possible solutions:
1. Implement an option that would prohibit for the higher value to be same as lower value (this would be useful anyway, consider price range selection in e-shop. You are not interested in items that have value precisely something.)
2. When both handles overlap, replace them with visually different object (that would look like handles stuck together). Its right side would work as right handle. Its left side would work as left handle.
We used slider in our project and we had quite many user complaints all resulting from confusion caused by this usability bug.