duplicate_named_argument

The argument for the named parameter '{0}' was already specified.

The analyzer produces this diagnostic when an invocation has two or more named arguments that have the same name.

The following code produces this diagnostic because there are two arguments with the name a:

void f(C c) {
  c.m(a: 0, a: 1);
}

class C {
  void m({int? a, int? b}) {}
}

If one of the arguments should have a different name, then change the name:

void f(C c) {
  c.m(a: 0, b: 1);
}

class C {
  void m({int? a, int? b}) {}
}

If one of the arguments is wrong, then remove it:

void f(C c) {
  c.m(a: 1);
}

class C {
  void m({int? a, int? b}) {}
}