javac throws duplicate warning messages on compiling the below program:

import java.util.Iterator;

public class TestUncheckedWarning {
public abstract class MyIterator<T> implements Iterator<T> {}

public abstract class Base<T extends Iterator<?>> {
protected abstract T foo();
public abstract Iterable<T> getIterable();
}

public abstract class Derived extends Base<MyIterator<Number>> {
public MyIterator foo() {
return getIterable().iterator().next();
}
}
}


javac -Xlint TestUncheckedWarning.java
TestUncheckedWarning.java:12: warning: foo() in TestUncheckedWarning.Derived overrides fo
o() in TestUncheckedWarning.Base; return type requires unchecked conversion
found : TestUncheckedWarning.MyIterator
required: TestUncheckedWarning.MyIterator<java.lang.Number>
                public MyIterator foo() {
                                  ^
TestUncheckedWarning.java:12: warning: foo() in TestUncheckedWarning.Derived overrides fo
o() in TestUncheckedWarning.Base; return type requires unchecked conversion
found : TestUncheckedWarning.MyIterator
required: TestUncheckedWarning.MyIterator<java.lang.Number>
                public MyIterator foo() {
                                  ^
2 warnings
testcase: java_util/generics/phase2/UncheckedConversionTest01