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When OpenAthens tells you there is already an account with the email address you are trying to use, return to the first step of the OpenAthens account creation window.
Uncheck the box that says “Login by email address,” highlighted here in yellow:
You should now be able to create the patron’s second account.
Unlike with their first account, where the patron can log into OpenAthens/Discovery using their email address as the username, subsequent accounts can only be logged into with the username we create (circled in red in the above image), which is the library’s prefix followed by the patron’s barcodeExternal system ID from their FOLIO record: [patron barcode]@[institution].edu.
Everything else about both the patron registration process, in both FOLIO and OpenAthens, and the patron experience remains the same for subsequent accounts as it is for first accounts.