You changed the weapon's FormID, but clicked "NO" when asked if you wanted to create a new form. This means, that all you did was replace the existing sword with your ID and other changes. Wherever the Ceremonial Dagger is used in the game, it will now be your Mehrunes Canine. This is fine, if that was what you wanted to do.
If you meant to create a new weapon, using the Ceremonial Dagger, so that both would now exist in the game, you should have created a new form.
Images would be nice, if you don't know how to do screen shots there are quite a few tutorials online and as for modding, this website really helped me alot http://cs.elderscrolls.com/index.php/Main_Page.
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If you meant to create a new weapon, using the Ceremonial Dagger, so that both would now exist in the game, you should have created a new form.
To learn about modding basics, start with: A beginner's guide, lesson 1 - The Construction Set Primer