Duplicate window is a feature that allows you to open multiple file tabs of a single file. When you create a duplicate window of a file, a new file tab is created with the file's name and a :2 is appended to it. The original file tab is appended with :1. You can continue to create duplicate windows, and each new window's file tab will be appended with an incremented number.
You can interact with and arrange duplicate windows the same way you would with any other window. Any changes you make in one of the duplicate windows will be reflected in all other duplicate windows of that file.
See also:
UltraEdit Wiki