If you don't target a window by index it will open a new window every time.
e.g,
tell application "Terminal"
    activate
    do script "ssh user@192.168.0.1"
end tell
or to deal with using existing windows on a fresh open.
tell application "System Events"
    if (count (processes whose bundle identifier is "com.apple.Terminal")) is 0 then
        tell application "Terminal"
            activate
            do script "ssh user@192.168.0.1" in window 0
        end tell
    else
        tell application "Terminal"
            do script "ssh user@192.168.0.1"
            activate
        end tell
    end if
end tell
more ideas here: http://hintsforums.macworld.com/archive/index.php/t-68252.html