If the duplicate files have names such as Copy of xxxxx.jpg then this is simply the result of selecting multiple files, using the CTRL key and slightly moving the mouse before the mouse button is released.
After the file is downloaded, double click the file to install.
Open TweakUI and go to the Mouse menu item. Change the "Drag" value to a higher number. I recommend a setting of 25 Pixels. This will allow the mouse to be moved slightly without any unexpected results when copying files.