The following macro definition compiles:

macro_rules! bad { ($a:expr, $a:expr) => {} }

Any attempt to match against this macro will result in an error, because the metavariable cannot be bound twice. There is no reason for the compiler to accept this definition as a result, since it always represents a bug, except for backwards compatibility. It should be fixed, either by assuring it won't break the ecosystem or by deprecating the behaviour and denying it the next edition.