From Project > Schedules > Schedules List click on the name of the Schedule you would like to copy (the row will highlight yellow).
Select the Copy button and update the Name for the new schedule:
Depending on the size of the schedule, the process may take a few minutes. Refresh this screen or review the progress using My Space > Overview - Background Tasks Admin Console Widget.
The copied schedule will be unpublished, remember to review the new schedule, check for any warnings, make sure any milestones / checkpoints appear as expected before publishing.