I ran into a problem where multiple users had the same externalSystemId. I submitted a separate issue for that in mod-users. Now I ran into a second problem. I cleared the externalSystemId values on all but one of the users to eliminate the duplicate values. However, I found that the FOLIO UI then set the values to "" (empty string) rather than setting them to null (removing the JSON property). This resulted in there still being duplicate values, only this time they were empty string rather than the original value. There needs to be a way to null out/remove values in that field. It's probably best to trim input values and then convert empty strings to null for consistency.