File duplicates in Adobe Photoshop
To create a copy of the file you are working on to protect the original from changes go to
Image > Duplicate. You can change the file name in a pop-up window, and then that new file will open for work and become active. You can continue to work on this copy or switch back to the original.
If you want to see the same file in two or more windows side by side in Photoshop, go to
Window > Arrange > New Window for ... (file name). Your changes will affect the file in all windows.
File duplicates in Adobe Illustrator
You can easily create a copy of your file in order to protect the original from changes, just go to
File > Save a Copy. But Illustrator will not open it automatically and you will continue to work with the original if you do not open the copy yourself. So be careful!
If you want to see the same file in two or more windows side by side in Illustrator, go to
Window > New Window.. This can be useful if you are working with different parts of a large file or want to see your image with different magnifications at the same time.