on_repeated

The type '{0}' can be included in the superclass constraints only once.

The analyzer produces this diagnostic when the same type is listed in the superclass constraints of a mixin multiple times.

The following code produces this diagnostic because A is included twice in the superclass constraints for M:

mixin M on A, A {
}

class A {}
class B {}

If a different type should be included in the superclass constraints, then replace one of the occurrences with the other type:

mixin M on A, B {
}

class A {}
class B {}

If no other type was intended, then remove the repeated type name:

mixin M on A {
}

class A {}
class B {}