This has been patched on 2b2t since decemember, it may work on other anarchy servers (l2x9)
Step 1. Make sure timer is 3+ speed
Step 2. Get on donkey.
Step 3. Toggle entitydesync
Step 4. hclip 80
step 5. timer on, take damage and break some blocks.
step 6. wait 10 seconds and untoggle entitydesync
step 7. it should have duped.