https://phabricator.wikimedia.org/p/dschwen/ says that this Phabricator account is linked to LDAP.

I cannot use the ldap login, because a user with the same email already exists. I'm deadlocked

Please provide a list of steps to reproduce the problem, with URLs and exact error messages.

Only partially sorted out in #wikimedia-devtools.
Initially https://phabricator.wikimedia.org/p/dschwen/ was only linked to the "LDAP user".
Trying to log in via LDAP, Phab says:

The email address ("wikimedia@.......") associated with the external account is already in use by an existing Phabricator account. Multiple Phabricator accounts may not have the same email address, so you can not use this email address to register a new Phabricator account.

@dschwen checked and could successfully log into https://wikitech.wikimedia.org/ (but had issues with that a few weeks back, and abogott fixed them. Also think "capital letter username issues" in Gerrit lately).

After @Aklapper triggered another "Welcome Email" for the user's Phab account, @dschwen got into his Phab account and could also attach his "MediaWiki user" account.

Then @dschwen tried to refresh his LDAP account in the Phab settings, triggering this error:

Your Phabricator account is already connected to an external account on this provider ("LDAP"), but you are currently logged in to the provider with a different account. Log out of the external service, then log back in with the correct account before refreshing the account link.

So the log in via LDAP still does not work, and it is unclear why (but at least @dschwen can comment directly in this task now).

but at least @dschwen can comment directly in this task now

Indeed I can. Let me know if I can do anything else to help debug this!