Entry for table field & already exists

What causes this issue?

You are attempting to select a data field that you want to use in the
results list of order selection as a display field. However, this field
has already been selected.

How to fix this error?

Delete your entry, or enter a different data field (one that you have
not yet selected). For a list of available data fields, use the
Possible Entries function (F4).

Error message extract from SAP system. Copyright SAP SE.