With regards to this, staff aren't obligated to give a winner another hat if they already have the one that was being hosted for. In this case, it may apply to the repeating winners as well. While it's not clear what exactly happened here, whether dual kicked him out of the event or something else happened that caused his character to be booted out, staff can choose whether or not they will give a replacement hat or let someone win twice. Most staff that don't intend to have made a note to announce it at the start of the event or in the event message since incidences of it occured.

There isn't one. It would vary on the staff member hosting but of course there will be a limit.