This simply cannot happen, because even if you installed first using an old installer and then the current installer, the new installer would have updated the Addon Manager location in the DLL.XML file, so it couldn't create a double entry.
Unless, you did the opposite, and installed the outdated version after you installed the current one, this would be the only possible case where you might see a double entry.
In this case, Uninstall everything from FSDT and every other product that might come with an outdated Addon Manager (Flightbeam, Qualitywings, etc.), then reinstall using ONLY the latest versions of everything from FSDT and Flightbeam.
If you must install the QW757, install it as the first thing you do, then all the FSDT/Flightbeam installers, using ONLY the latest installers.