There is no warning when renaming generic to identifier which already exists.

Example:
1) have a class:
public class Main<X,Y> {

    X a;
    Y b;
}

put caret on X and call Rename
-> it is possible to enter Y w/o any warning.
-> the names of imported classes are not valid names, since it can break the code

Example:

rename X to List in following code:

import java.util.List;
public class Main<X> {
    List a;
    X b;
    public void test() {
        a.add("");        
    }
}