The Image menu contains commands that use or affect the entire
image in some way, not just the active layer or some other
specific part of the image.
This command can be found in each image menu at
->.
Duplicate creates a new image which is an exact copy of the
current one, with all its layers, channels, pathes. The GIMP
clipboard is unaffected.
The Ctrl+D key combination
will call the Duplicate function.