Error "User has already registered"

In the instance where a user has gone to register but didn't complete the process they may get the error 'xxx has already registered, xx only allows for one registration'. 

This may be a caching issue.  The user should use a PC to login and register. If you were already registering on a pc, your browser needs to be reloaded.

​In Chrome that would mean Right Click > Reload

​In Safari that would mean selecting Option + Cmd + E