After all this years, this is still not working! Each time I double click on a file in Windows Explorer or in my version control system a new instance is launched, which takes at least a minute in my case. This is by far the MOST ANNOYING problem of PL/SQL Developer!
Marco, how can it be a Windows Explorer problem? The Explorer knows nothing about PL/SQL Developer, the explorer can only launch a new instance with the given file. It is up to the new instance to transfer the file to an existing instance (or not doing it, depending on a user preference). This should be really no problem for an accomplished programmer.