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SAML/LDAP login: An account with that username already exists

Summary
SAML login: An account with that username already exists

Steps to reproduce
Mattermost Version 5.37.2
ADFS 2019 server - identity provider
* Relying Party Trust set up as per:
Configure SAML with Microsoft ADFS using Microsoft Windows Server 2016 — Mattermost documentation
Operational LDAP integration.
LDAP ID Attribute = ObjectGUID
Authentication Service on user account = LDAP

Expected behavior
The useraccount should be logged in using the SAML button on https://mattermost.domain/login

Observed behavior
Mattermost displays “Error” An account with that username already exists.
If I change the authentication service on the useraccount to SAML the login works perfectly.

I think the problem is that I use ObjectGUID as the LDAP ID Attribute and im not able to pass the ObjectGUID as a claim type from ADFS.
Is it possible to have the authentication service to LDAP and still get SAML to work?

BR
Sverre