XML tag ID 4 that was originally declared as is being redeclared as

Error message:

XML tag ID 4 that was originally declared as Aaaaa is being redeclared as Bbbbb.

Cause and solution:

Each group of data should be declared in other tag.

In this case, there were 2 groups with tag 4: [Aaaaa!4!CollectionID] and [Bbbbb!4!CollectionID].

The solution was to create new tag to the new group: [Bbbbb!13!CollectionID].

Of course that I had to add new UNION select in order to select the data of Bbbbb and to assign it to a parent.