From the standpoint of ASP.NET, browser tabs do not even exist, there are only Web pages. Where to display a page, in what windows — is a sole responsibility of a client side and specific to a particular browser.
You don't have any control over it. Come to think about, this is good: the user knows better whether a click replaces the page or opens it in a new tab or a separate top-level window. No need to interfere with this optional behavior.
See also this answer by That's Aragon:
Problem in more then one tab opening[
^].
—SA