Expression for &1-&2 (&3) already exists

What causes this issue?

You have tried to create an expression that already exists in this
form.

System Response

The system informs you that you are trying to create functionally
identical expressions.

How to fix this error?

Check whether you can use one of the existing expressions.
If you still want to create a functionally identical expression, this
is possible. You might want to do so if you want to use the
documentation in a different way than is the case with the existing
expression.

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.