1. Copy something to the clipboard that is not text (eg an image)
2. Place a command block in the world
3. Click the paste button on the command input field. Nothing happens.
4. Outside minecraft, copy some text onto the clipboard
5. Return to minecraft, click the paste button on the command input field again. Notice that the text is pasted twice.
If you repeat, but click the paste button multiple times in step 3, it will paste that many extra times in step 5.