I don't think this would be better. Then every time a new instance would be started which works indepedent of the existing one. This is especially unconvenient when opening files via the .desktop items (task bar, app menus, file managers, ...).
To me it sounds rather like this would only fix the symptom for another problem: why does Geany think the socket it wants to use belongs to another user?
In any case, you always can override the .desktop yourself via `~/.local/share/applications/geany.desktop`.