Collision of fixed exchange rate in document &1

What causes this issue?

Document &V1& comes from SAP Transportation Management (SAP TM).
You attempted to assign a TM document &V1& with a fixed currency key
&V2& and exchange rate &V3&. This conflicts with another TM document
that has a different fixed currency or exchange rate and has already
been assigned.

System Response

The assignment of this TM document is no longer possible.

How to fix this error?

Divide the invoice into parts, grouping matching currency and exchange
rates together.

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.