dbcc checkdb('XYZ', REPAIR_ALLOW_DATA_LOSS) indicating that XYZ is already open and can only have one user at a time

Revert the database back to multi user mode and try something like the following.

alter database xyz SET single_user with rollback immediate 

dbcc checkdb('xyz',REPAIR_ALLOW_DATA_LOSS)

Likely another process is grabbing the database connection before you get in there. Combining both statements should ensure you get that connection.