I'm sorry that Dreamweaver and Extension Manger are acting up on you with our extensions installed.
If you uninstalled Dreamweaver, then reinstalled it and there are still extensions listed for Dreamweaver, then the user configuration folder did not get removed when the uninstall took place. It should have been removed, or at least, you should have been asked to remove it. It may also be the case if you installed some extensions when logged in as the system Administration user, and some when logged in as a different user.
Sounds like you should take a look at some of the options on this page:
In particular item 4. Item 12 may help as well if 4 doesn't. 12 will cause all extensions to be removed from within Dreamweaver. For #12, if you ever have installed an extension when logged in as the Administrator user, then you may have to check the "all users" folder for a copy of a Dreamweaver configuration folder, on Mac, it may be within the root folder.
If you are still having any issues, please post a support incident so that someone can take a look at your system to see what is going on.