error[E0428]: the name `A` is defined multiple times
  --> $DIR/define-two.rs:15:21
   |
LL | #[proc_macro_derive(A)]
   |                     - previous definition of the macro `A` here
...
LL | #[proc_macro_derive(A)]
   |                     ^ `A` redefined here
   |
   = note: `A` must be defined only once in the macro namespace of this module

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0428`.