Can anyone tell me why my iMac has suddenly started showing my macbook desktop instead of the original desktop? Original desktop shows as a folder

Do you have iCloud Dfrive enabled on both machines with Desktop and Documents enabled? Also do both machines use the same iCloud account?

This is a longshot but check to see how they are configured.

Your questions is lacking any useful information so we would have no idea. Please carefully look over Writing an effective Apple Support Question and then re-write your question so you leave some clues. We have no idea what you have done, what Mac you own, what year, the version of Mac OS or anything.

Not sure what you mean. The Desktop never showed as a folder with any version of macOS or OS X. Maybe you could explain further?

Are they connected together for Target Display Mode, Target Disk Mode, Remote Desktop or Screen Sharing?