As writing this article, it is possible to have a loanee with the same email address as a sub-user in Reftab. This can usually occur after a bulk import or after a SCIM sync. In a future update to Reftab, this will not be possible in order to avoid duplicate entries.
How to find users with duplicate email address:
Click โReportsโ > โNew Reportโ. Give the report a name and select โShow Loaneesโ and click โRun Reportโ.

Next, when results appear click โChart Builderโ and select โTable Heatmapโ.
Then, drag โLoanee Emailโ to the left side. You will see a chart expand to show a count of email addresses that occur. In the screenshot below, I can see there are 2 records of โhugo@reftab.comโ. This will be a loanee I will merge with the user account.

Note that you can right click on the chart and copy it into another program to easily refer back to it.
How to merge loanee with user:
Next, click โLoaneesโ in the green navigation bar.
Use the filter to search for the duplicate user. In this example, I search for hugo and I see two results appear.
Click the Edit button on the loanee.

On the window that appears you will see โConvert to User Accountโ. Notice the text that says โIf a user already exists with same email, the two will be merged together.โ
Note that if the loanee has loaned items, they will also be moved into the user.

The process is complete. On the loans page youโll now only see one user.

Reach out to help@reftab.com for assistance.