Can we use same icon image for more than one ios (ipad/iphone) applications? Is there any such rule that each application should have a different icon?
Thanks.
There is no such rule, as far as I know. Indeed there are apps in the App Store that have the same icon, and others with very minor variations (e.g., the color tone).
Apart from requirements about the size (of the different icon versions you have to submit) that are relevant for approval, the only thing that you should take into account (but this is not relevant for approval) is that your app icon is attractive, reflect some how the app functionality, and allows your app to be easily recognized.
If your unique icon design satisfies this principle for you, then it is fine.