3.5. Copying an Existing Event
To Copy an existing event, press the Edit button, and then press the "Copy Event" button instead of the "Replace Event" button. Note that you can make changes before copying, effectively editing and copying an event in one step. This will leave a copy of the original event as is and create a new event with the new details.
You can also Copy an event to other calendars, if Adding to Multiple Calendars is enabled. Edit the event as above, and select the calendars from the calendar list. (Make sure to unselect the current calendar, or you'll get another copy there, too.) Then, press the "Copy" button.