Last modified: 2014-05-09 23:32:28 UTC
If user is saving the page as redirect to another page which already is redirect, he should get some kind of confirmation dialog like:
Warning: Double redirect creation.
The target [[Foo]] is already redirect to [[Bar]].
[Fix the target] [Save anyway]
Note that a more frequent source of redirects is going to be changing an existing page (by move or manual redirect) to which other pages already redirect.