The field "XXX" was renamed conflicting with an existing field name

[e_mariee]

I am running a workflow to format my inputs. However, I get this error (The field "XXX" was renamed conflicting with an existing field name). What does this mean? The specific field it is referring too is not duplicated nor empty. I am not sure why it is not capturing this specific field?

[wildflower]

We upgraded to Designer 2021.3 from 2019.4, I noticed that the Select tool is now case sensitive. I also noticed that a union, join or append tool will rename the source field, thus breaking the downstream Select tool. Was never case sensitive before.

E.g. Source field is QUALIFYING_SOURCE. A union tool renames it to Qualifying_Source and causes an error: "Select (31) The field "Qualifying_Source" was renamed conflicting with an existing field name" - the tool will show a conflict with the field, listing it twice.  This is a headache for us - the Select tool only breaks if we click into it, revealing the conflict. Forgetting the fields in the select and bringing the fields back in (with whatever case they're supposed to be) resolves the issue.