That could happen if you accidentally invoked the "duplicate" shortcut in Finder with the desktop folder selected. Over time, the two folders contents would diverge.
To make sure everything in the duplicate is already in the main Desktop folder, open the Desktop folder in a Finder window, and move it to the left side of the display. then choose new Finder Window from the Finder's File menu, and open 'Desktop copy' and place it on the right side of the display. I prefer view as List, by name alphabetical.
now go down the list of files, and make sure everything in the duplicate also appear in the main folder. in case of two different-sized or different-dated files, you will have to choose which to keep.
Once Desktop has all the files also present in Desktop copy, you can delete Desktop copy.
On a Mac using APFS File system, the amount of space reclaimed will be extremely small, because most duplicates are not actually duplicated data blocks, but use a pointer to the original instead.