when an interface is changed using copy->paste->edit sync suggests to update 
the copied method in the implementing class instead of adding a new method.

to reconstruct:
1. create an interface:
public interface A {
    public void func();
}
2. create a class:
public class B implements A{
    public void func() {
    }
}
3. in A, copy the declaration of func(), and paste it in the next line. then, 
change the name to func2(). now you should have:
public interface A {
    public void func();
    public void func2();
}
4. wait for the sync
results: sync suggests to update the copied method in the implementing class 
instead of adding a new method.

note: the same happens when the method is typed. I guess it has nothing to do 
with the copy/paste rather than the parsing of two identical method declarations