External BP number &1 has already been assigned for business partner &2

What causes this issue?

A business partner was created with an external BP number that is
already being used for one or more business partners.

System Response

If you create a business partner in the <ZH>BP dialog</> with an
external BP number that is already being used, you can still save the
business partner's data.
In this case the system issues this message as a warning.
Note
The following information is not relevant if you are using a CRM system.
If you <ZH>create</> a business partner with an external BP number using
<ZH>Direct Input</> in an ERP System and the external BP number is
already being used for one or more business partners, the business
partner's data is still saved.
In these cases the system issues this message as a warning.
If you <ZH>change</> a business partner with an external BP number using
<ZH>Direct Input</> in an ERP System and the external BP number is
already being used for one or more business partners, this message is
issued as an error.
This is the case if the field business partner (BU_PARTNER) is not
filled, and the external BP number (BU_BPEXT) is referred to as an
identifying characteristic.
You must state the business partner number (BU_PARTNER) in order to
change the business partner in the Direct Input.

How to fix this error?

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

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.