Interval & & & & already exists

What causes this issue?

When inserting an interval, its ready existence was determined.

System Response

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

How to fix this error?

Enter another number range number. If the intervals are distinguished
by 'To' years, enter another 'To' year.

Error message extract from SAP system. Copyright SAP SE.