This is for sims3pack custom content?
I believe the point of this procedure is to remove/uninstall the items through the Launcher so that they are no longer represented in the dbc files in the DCCache folder, which is where Launcher installed custom content lives. Otherwise yes, you will have installed the content twice. It doesn't matter if the packs show up on the Launcher to be installed, that would mean they are still in the Downloads folder but there is no reason for them to remain there. The game itself does not read the Downloads folder, only the Launcher does.
