Field & is used in several set lines

What causes this issue?

You have tried to enter sub-sets for the same table field in several
set lines in a multi-set. However, the sub-sets of a multi-set must
each belong to different fields ("dimensions").

System Response

The system informs you that the field cannot be used twice.

How to fix this error?

You must correct this double usage of the field before storing the set.

Error message extract from SAP system. Copyright SAP SE.