error[E0254]: the name `core` is defined multiple times
  --> $DIR/unresolved-extern-mod-suggestion.rs:2:5
   |
LL | extern crate core;
   | ------------------ previous import of the extern crate `core` here
LL | use core;
   |     ^^^^ `core` reimported here
   |
   = note: `core` must be defined only once in the type namespace of this module

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0254`.