Collision of fixed exchange rate for contract &1

What causes this issue?

You are attempting to assign contract &V1& with fixed currency key &V2&
and exchange rate &V3&. This conflicts with another purchase document
that has a different fixed currency or exchange rate and has already
been assigned.

System Response

The assignment of this contract is no longer possible.

How to fix this error?

Split the invoice into several parts, according to currency and exchange
rate specifications.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.