Error in data transfer, extension of rebate arrangements (internal error)

What causes this issue?

The condition data could not be placed in an internal table because an
entry with the same key already exists.
The rebate arrangement concerned has the number &V1&. The error
occurred in function module &V2&. It may be due to one of the following
reasons:

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

internal program error
faulty call of function module &V2& with invalid or inconsistent data

System Response

How to fix this error?

Contact your system administrator.

Error message extract from SAP system. Copyright SAP SE.