Field name & not unique

What causes this issue?

When you define aggregates (views, lock objects or matchcode objects),
you can assign the aggregate fields your own names (the system assigns
names to the basic fields automatically). These names must be unique.

System Response

The system has established that the names assigned to the aggregate
fields in an aggregate definition are not unique and issues an error
message.

How to fix this error?

Assign unique names to the aggregate fields in question.

Error message extract from SAP system. Copyright SAP SE.