I was recently comparing a new design to a former design and had two Altium windows up on two monitors. I did this by dragging a tab off of my main Altium. Now, whenever I double click a file under Projects, it opens in a new window. Then I have to drag the tab back to the main window and close the new Altium window and select only close that window. This is getting annoying.
Just typing up this, I did one last check of all Preferences and figured out that "Remember Window for each Document Kind" was the culprit. This is the forum equivalent of figuring out the answer just after you ask someone. Anyhow, if it happens to someone else...