FULL PRODUCT VERSION :
java version "1.6.0_02"
Java(TM) SE Runtime Environment (build 1.6.0_02-b05)
Java HotSpot(TM) Client VM (build 1.6.0_02-b05, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
javap shows 2 checkcast instructions for my example Test.java compiled with JDK 6 compiler.

Test.java:
public class Test implements Externalizable
{
private Object[] centerData;

public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
centerData = (Object[])in.readObject();
}

'javap -c' output for readExternal():
public void readExternal(java.io.ObjectInput) throws java.io.IOException, java.lang.ClassNotFoundException;
Code:
0: aload_0
1: aload_1
2: invokeinterface #2, 1; //InterfaceMethod java/io/ObjectInput.readObject:()Ljava/lang/Object;
7: checkcast #3; //class "[Ljava/lang/Object;"
10: checkcast #3; //class "[Ljava/lang/Object;"
13: putfield #4; //Field centerData:[Ljava/lang/Object;
16: return
 

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
compile example Test.java
run javap -c Test

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
checkcast instruction appears only once
ACTUAL -
checkcast instruction appears twice

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.io.*;

public class Test implements Externalizable
{
    private Object[] centerData;

    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
        centerData = (Object[])in.readObject();
    }

    public void writeExternal(ObjectOutput out) throws IOException {
        out.writeObject(centerData);
    }

}
---------- END SOURCE ----------