Just got dced, and i cant log back in it keeps saying “character with this name already exists” even though i was just playing him. Another monk messaged me and said the same thing.
Update: logging out completely and back in seemed to help
Same, except i cant get back in. stuck on the loading boss 
Usually, logging into another toon first and then back on your main toon (the one thats causing an error) tends to work.
But you can also try this
https://us.support.blizzard.com/en/help/product/wow/197/834/solution
Unstack your character