When compiling the following code, duplicate entries are created in the Constant Pool:

Vector v = new Vector();
Iterator iter = v.iterator();
while (iter.hasNext()) {
   Object o = iter.next();
   Object o2 = o;
}
for (Object o: v) {
   Object o2 = o;
}

From the constant pool (extracted by JRAW):
...
cp4: Method #cp2 #cp29 // java.util.Vector.iterator () -> java.util.Iterator
...
cp7: Method #cp2 #cp29 // java.util.Vector.iterator () -> java.util.Iterator
...

Javac 1.5 beta, build-51
###@###.### 2004-05-27
I just noticed this for a simple program with a try block:

public class TT {
public static void main(java.lang.String[] unused) {
  try {
    System.out.println("try");
  } catch (Exception e) {
    System.out.println("catch");
  }
}
};

    class #25; // #5
    class #25; // #16
    Utf8 "java/lang/Exception"; // #25

While the duplicate constant pool entries might still be valid class file,
this can't be good for download size.