Various varieties of Outlook and Outlook Express have a peculiar habit of repeatedly updating the clipboard with the same thing, over and over. The effect, depending on the collection settings in ClipMate, is that you end up with the same clip captured over and over, or you hear the "clip rejected" sound, over and over. And it does NOT correspond to copy/paste operations.
What seems to be happening, is that when you select some text in an Outlook article, and copy it, and IF you leave the original text highlighted, then Outlook will continually re-copy the information to the clipboard. It'll do it when you switch to another article or message, for example. As you open the other articles, Outlook will send the data to the clipboard again. Why, is anyone's guess. But to prevent it from bothering you, be sure to un-highlight any text that you copy. i.e. highlight, copy, and then click somewhere else, to let Outlook know: "no, I don't want that thing copied to the clipboard over and over".
Some versions are even more persistent. If un-highlighting doesn't help, then simply go into ClipMate and select any other clip. Or, copy anything from any other application. In either case, the clipboard is overwritten, causing Outlook to give up on its quest to keep refreshing the clipboard contents.