Assignment not valid; Cover pool contains allocated account assignment

What causes this issue?

You want to assign expenditures cover pool &V3& to revenues FM account
assignment &V1& as the receiver of additional revenues increasing the
budget.
Revenues FM account assignment &V1& is however already assigned to
expenditures FM account assignment &V2& as the receiver of additional
revenues increasing the budget. The account assignment is a member of
the cover pool.
An expenditures FM account assignment cannot be assigned more than
once.

System Response

The program cannot process this entry.

How to fix this error?

Enter another expenditures cover pool or delete this account assignment
from the cover pool.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.