Warning for element have annotation Annotatedfor(...) and UnannotatedFor(...)

[Ao-senXiong]

Issue a warning if one element(package, class, method ...) have both annotation with one of the value matches.

[wmdietl]

Let's widen this issue to cover all defaulting annotations.
For example, this does not produce a warning:

import org.checkerframework.framework.qual.DefaultQualifier;
import org.checkerframework.framework.qual.TypeUseLocation;
import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.checker.nullness.qual.NonNull;

@DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.IMPLICIT_UPPER_BOUND)
@DefaultQualifier(value = Nullable.class, locations = TypeUseLocation.IMPLICIT_UPPER_BOUND)
@DefaultQualifier(value = Nullable.class, locations = TypeUseLocation.IMPLICIT_UPPER_BOUND)
class Demo<T> {
  void foo(T p) {
    p.toString(); // OK?
  }
}

Nullable is listed twice with the same locations, which should produce a warning.

The same location is used with different qualifiers from the same hierarchy, which should also be a warning.
In my tests NonNull always won and the .toString never produced an error, even with different orders of the annotations.