Have a look at your database for ISPConfig.
In phpMyAdmin, fist select the ISPConfigdatabase (default= ispconfigdb) then go to isp_isp_user table and check your user records.
If you find the user which must be deleted, delete it by pressing the red cross.
Warning: be carefull do not screw your database!
At the other hand, if you can't find the user within the database it does not excists there.
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