An event registrant can have additional registrants can act as a host and have guests. However, when the additional guest registrant is added with a different registration option, that registration shows as an additional registration on the hosts constituent record, making it look as if he is registered for the same event twice.
We're currently evaluating this issue for a fix in a future patch or service pack.
Steps to Duplicate
Go to an Event
On the options tab, make sure there are least 2 Registration options (or add them)
Go to the Registrations tab
Enter a name in the Registrant field
Under Registration option, choose one from the drop down
Choose the Registrant
Go to a second line and choose the a different Registration option
For the next Registrant, choose a different name from the drop down other than the Registrant from above, or choose Unnamed guest
Go to the original registrants constituent record
Go to the Events tab
Observe that there are two Registrations for this constituent