SAP Message USMD1090 - Entity type &1, relationship &2: Leading entity type &3 is not unique

Diagnosis The From-entity type &V4& has the leading entity type &V3&, whichneeds to be unique in the context of the To-entity type &V1&:V3 must be either a part of the key of v1 or must be assignedthrough exactly one relationship of the type Referencing to theentity type &V1&. System Response The data model cannot be activated. Procedure Adjust the data model so that the condition described above isfulfilled.