Hi,
(For other users' knowledge) Ctrl+click does this using Yahoo!, Google and other search engines as well can be done in the Chrome browser, Firefox, IE, etc... This is a shortcut to open another tab with that search.
That said...
If you mean it's opening two EXTRA tabs, the only time I've seen this is if some use the middle mouse button for clicking, (Some mice work differently) they are double clicking, (ONLY NEED TO CLICK ONCE) or the mouse click settings are too touchy or need to be adjusted. (Updating mouse drivers helps as well)
A browser reinstall may also work, of course backing up your settings and bookmarks first.
As always, a scan for malware \ browser hijacks, would be a good idea too.
I could not produce the issue myself and if not a mouse issue or if a browser reinstall won't work, if no malware (infection) then not sure and maybe someone else knows.