double-click automatically creates copy of a file

today my MacBook started automatically making copies of files when I double-click on them. So if I double-click a file on my desktop (such as a photo) rather than just opening the file, which is what i want to happen, it opens the file and creates a copy of the file on the desktop. this is annoying! it happens with files of all types. is there some setting that i can change? or is something wrong with the machine?