Parallel execution may result in duplicate error messages

[Zoxc]

When executing parallel blocks we'll reveal more error messages because we don't immediately stop at the first fatal one. This may lead to duplicate error messages however. For example src/test/ui/issues/issue-21946.rs the from test suite:

trait Foo {
    type A;
}

struct FooStruct;

impl Foo for FooStruct {
//~^ ERROR overflow evaluating the requirement `<FooStruct as Foo>::A`
    type A = <FooStruct as Foo>::A;
    //~^ ERROR overflow evaluating the requirement `<FooStruct as Foo>::A`
}

fn main() {}

Produces:

error[E0275]: overflow evaluating the requirement `<FooStruct as Foo>::A`
  --> $DIR/issue-21946.rs:7:6
   |
LL | impl Foo for FooStruct {
   |      ^^^

error[E0275]: overflow evaluating the requirement `<FooStruct as Foo>::A`
  --> $DIR/issue-21946.rs:9:5
   |
LL |     type A = <FooStruct as Foo>::A;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0275`.

We should look into deduplicating these error messages.

[estebank]

We only emit one of the errors now.

error[[E0275]](https://doc.rust-lang.org/stable/error_codes/E0275.html): overflow evaluating the requirement `<FooStruct as Foo>::A == _`
  --> src/main.rs:11:14
   |
11 |     type A = <FooStruct as Foo>::A;
   |              ^^^^^^^^^^^^^^^^^^^^^