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Joomla! itself prevents two different users from having the same numeric user ID, same username or same email address. In fact this is enforced at the database level.

Regarding downloads, no. You cannot limit what the web server will serve to each "user" unless you write, for example, an Apache module which is way outside the scope of Admin Tools or what we do as a company.

If you are talking about downloads from a particular component you have installed on your site the answer is again no. User limits are not security parameters. Therefore it's out of scope of Admin Tools. You can ask the developer of your download extension if they are interested in implementing this kind of feature. Do note that your question is very open ended, so if you do ask the extension developer you have to at least specify:

• What is a "user"? A logged in user?
• How do you handle non-logged-in users? Do you apply a download limit to all of them as a single group? Or do you apply limits by IP? If the latter, nobody will bother doing that because you can disconnect, reconnect, get a new IP and off you go again (spoken from experience)
• What is a "day"? A 24-hour rolling window? Midnight to midnight? In the latter case, which timezone: GMT, the site's, the user's (if they bothered defining, also can be used to circumvent the limit) or an arbitrary one? Have you considered the confusion of users who live in a different timezone than the default being used?

As an extensions developer who has pondered all these questions and writes his own download software I can tell you that download limits make zero sense. They get in the way of legitimate users and will not dissuade miscreants who download everything they can before filing a bogus chargeback request with the payments service provider.

The whole concept of limited daily downloads is a remnant of the BBS days where you only had so many modems and you needed people to only be logged in for a limited amount of time to make sure everyone could use the service. This access model died circa 1994 with the advent of the World Wide Web and the ensuing widespread Internet access - even if it was a meager dial-up connection.