I have two Addon Managers listed within FSX
This because you installed something using an old FSDT installer (one that uses to install into the sim folder)
after having installed something using a new installer (that installs outside the sim folder) and you installed the old stuff AFTER the new FSDT installer migrated all your files to the new folders.
Basically, installing a new FSDT installer AFTER you installed something using an old installer, it's OK, since the new installer will move whatever needs to be moved into the new locations.
But doing the opposite (installing with an old installer on top of a new one), will never work and result in two Addon Manager, and of course there's nothing we can do about this, since we cannot retroactively change old installers you already downloaded, and those do not know anything about the new installation folders.
Try to download and install the latest Stand-Alone Addon Manager, since it should be able to clean up all the things installed using the old installer and remove the double Addon Manager.
If that doesn't work (it should), uninstall *everything* from FSDT, reply YES to the questions about removing the Addon Manager, and reinstall everything using ONLY the CURRENT INSTALLERS.