error[E0119]: conflicting implementations of trait `Foo` for type `Bar`
  --> $DIR/coherence-constrained.rs:17:1
   |
LL | impl Foo for Bar {
   | ---------------- first implementation here
...
LL | impl Foo for Bar {
   | ^^^^^^^^^^^^^^^^ conflicting implementation for `Bar`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0119`.