IOS – How to show app ad’s only to new users

That's a really interesting question.

But first I give you another situation: If user 2 wants to use an app already bought and downloaded by user 1, the only thing user 2 needs to do is add the .ipa file of that app in his iTunes and authorize his computer with the apple id of user 1. (in iTunes click on 'Store' > 'Authorize this computer')

If two people want to use the same app they don't need to both download the app. They don't even have to both purchase the app. They can share the .ipa file if they authorize each other's computers.

In the situation you present - where user 2 also bought the app but didn't download it - I think he would still need to authorize his computer with the apple id of user 1 in iTunes. That's because I think the identity (apple id) of the buyer/downloader of the app is stored in the .ipa file, but I'm not 100% certain about that.

I'm not too sure as to what caused the problem but all you need to do is go to:

Settings>General>Reset>Reset All Settings

This will result in losing all the wifi passwords you connected to but you can just retype them. Then just update or download something (that's free of course like an already purchased app or an update), type in the password, cancel (unless you actually want to download it) and now you don't need your password anymore.