If starting Kate from the KDE menu use the Menu Editor to edit the Kate's "Command" text box. Insert '--use' between 'kate' and the '%U'. It should end up looking like this: Command: kate --use %U
You also need to edit this file: /home/$USER/.local/share/applications/kde4-kate.desktop and change the "Exec: " line to: Exec: kate --use %U
OK. Just did both. Seems I now have a single window/session. Much better than before. :) Yet now when I have Kate open and select a file from within Konqueror, the file opens in Kate but Kate will not snap to the front and gain focus. Kate flashes in the task bar/panel.
I changed the focus stealing prevention level from Low to None to get Kate to move to the front. I don't know how changing that option will affect other things I do.
I have that option set to Low for years and Kate always came forward. Please don't say "bug report." :)
I use a Kate icon in the toolbar panel. To get it to work correctly I had use the choice "configure Kate button" that you get when you right click over the icon. Then go to >> "Application" tab >> "Advanced Options" and change "DCOP registration" to None. My icon setup for calling Kate is not typical so you may not need to do this.
OK, I figured that out after I posted. Just create my own button. :)
Thanks!
Darrell