Display settings reset every time I turn on iMac

[macmee]

Every time I turn on my iMac the display settings go back to how they used to be, and I have to change them back to how I want them. I use my external 24" monitor for my primary monitor, and use that as my primary monitor, and have the iMac as a secondary to my right. However I boot my computer and my iMac is again the default monitor and it puts my secondary screen on my left. I have to change it every time I boot up.