I don't think it does this already, at least not that I've found. 

Anyone used to the OSX dock should understand the behaviour I'm after.

Basically I want to show running items in my dock, but ONLY if they haven't already been "docked".

e.g. I have an empty dock. I run thunderbird and obviously it's icon appears in the tasks area in my dock.

If I right click and "Add This Application" to the dock I currently then have 2 thunderbird icons: One "docked" with a running indicator below it, and another in the tasks area. This is what I DON'T want. If the icon is docked, then there is no need to duplicate the icon on the tasks area.

Hopefully that makes sense

Thanks

Bryan