How do I sync apps on my itouch on another computer without losing the apps already on my itouch?

How do I sync apps on my itouch on another computer without losing the apps already on my itouch?
I have an itouch 3g and I have downloaded and bought apps that are on the itouch, but when i download apps from another computer, that i have authorized, and try to sync them a warning comes up saying “Are you sure want to sync applications? All existing applications and their data on the iPod “Blank’s iPod” will be replace with applications on the iTunes library?” How do i prevent this? Please help.


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