SAP Message REBDCA054 - &4 is already assigned to two arch. objects (&1 and &2) in period &3

Diagnosis
&V4& has been assigned to two architectural objects in period&V3&: &V1& and &V2&.
This is not permitted.

System Response
The current assignment is not possible due to an existing assignment.

Procedure
Decide which of the two assignments you require.