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Thank you for the question. You can disable multiple desktops by going to Mission Control and clicking the X over the multiple desktops. To do this, click the search button on the very top right corner of your screen and type Mission Control. Double click or press enter to open Mission Control. This will show the other desktops on the top of the screen. Hover over Desktops 2, 3, 4 or however many you have, and click the X over the desktop.
This may not completely prevent applications from opening in multiple desktops until you set them all to open in the same desktop. To do this, hold control while clicking an open application in your dock, then click Options -> This Desktop. That will make it so the application always opens in your primary desktop instead of an alternate desktop.