Itemupdating event not
Instead of writing more paragraphs I can use tabular to explain, I think this will be easy to understand.
Each event method has a SPItem Event Properties parameter named properties.
You could think that the current values are stored in the Before Properties of the item but that’s not true: the Before Properties are unreliable at this point. When the name of the planet is changed, the update is canceled and an error message is returned to the user.
If the name isn’t changed, the changes are saved to the Share Point list.
It appears as though they are firing twice in this situation because Share Point is updating the properties on the document and then checking it in on the same request.
But that option exists to be used, and some people really do need it.
This walkthrough is based on the first one where I have a custom list with planets of our solar system.