These urls all claim different canonical urls
https://en.wikipedia.org/wiki/San_Francisco
<link rel="canonical" href="https://en.wikipedia.org/wiki/San_Francisco">
https://en.wikipedia.org/w/index.php?title=San_Francisco
<link rel="canonical" href="https://en.wikipedia.org/w/index.php?title=San_Francisco" />
https://en.wikipedia.org/w/?title=San_Francisco
<link rel="canonical" href="https://en.wikipedia.org/w/?title=San_Francisco" />
https://en.wikipedia.org/?title=San_Francisco
<link rel="canonical" href="https://en.wikipedia.org/?title=San_Francisco" />
They are exactly the same resource though - so which one is the real canonical URL?