In the case, it is MS Expression Web 3.
I think it needs DDE to open pages in tabs. The system's default "Open" action uses DDE.
When the main exec is called, it opens a new instance and it takes much longer time than opening a new tab.
WinSCP does not support DDE. What editor are you using?
How to add an Editor that uses DDE options to open files?
I haven't found a way to do so.
Some editors need it to work properly, unfortunately...