Iam downloading OS X Mountain Lion to my iMac, do I have to purchase this again for my Macbook Air?
As long as both Macs are on the same iTunes account you can just download it again from MAS.
No, after it dl's quit out of the installer. Go to /Applications and copy the installer to a pocket drive, another hard disk or the other computer. Then, double-click installer to begin the install.
How to install Lion or Mountain Lion on multiple computers
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