Open your servername_charname_eq2_uisettings.xml file. Do a search for "MainChat." Each time you find that it corresponds to a potential chat window, either one actively on screen or one off screen.
First, look for one called MainChat_clone_0. See if it has any lines that being with <ChatTab underneath it -- I'm willing to bet it doesn't.
If it doesn't, then look for a different MainChat line that has at least one <Chattab line underneath it and change the number after clone_ to 0 and replace the other one (that used to be 0) with the number you just removed.
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