Duplicate Key Values in Dynamic List

I receive the following error message when I change view of my dynamic list to Tree:

Duplicate key values found in column "Ref". Unable to display data in a list.

Other modes work fine. Where the problem is?

You have several lines in your query result with the same value in Ref column. They should be unique. When you see in other view the query result is filtered by folder and in tree view the filter is different and one (or more) items with the same Ref value occur in the result. You need to change the query to guarantee that the Ref column is unique.