Generally it's added to, but sometimes there are "rollup" updates that are a collection of previously released updates into a single package.
This really isn't something you need to worry about. If an update supersedes a previous one, it will overwrite the necessary files, so there is a very minimal amount of wasted space.
It's also generally best to lose a little disk space rather than risk exposing some flaw in Windows, or a Windows program, that is being actively exploited.
When I check my computer for downloaded updates, it list some of the same things downloaded in different months. My question is, If I receive a note to download a new update, does it take the place of the old one in my computer or is it added to my computer. is it safe to remove the old up dates listed.
Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic