No warning when the same rule is added twice

[francoisthire]

No warning are printed on the following example:

A : Type.

a : A.
b : A.

def f  : A -> A.

[] f a --> b.
[] f a --> b.

In the more general case when a rewrite rule is added and will never be used it might be interesting to warn the user when it is possible.

[francoisthire]

@Gaspi @GuillaumeGen