System &1 already registered in domain

What causes this issue?

A system <LS>&V1&</> is already registered in the domain controller of
the transport domain <LS>&V2&</>. Two systems with the same system
names cannot be administered in one domain.

System Response

You cannot include the system in domain <LS>&V2&</>.

How to fix this error?

Use transaction STMS to display the systems which are already
registered (STMS -> Overview -> Systems) at the domain controller.
Delete system <LS>&V1&</> from the transport domain and repeat the
logon in system <LS>&V1&</> if you want to register the new system
<LS>&V1&</> in the transport domain <LS>&V2&</>.

Error message extract from SAP system. Copyright SAP SE.