Try find conflicting UIDs

src/main/java/net/minecraft/server/WorldData.java

@@ -1,6 +1,7 @@
 package net.minecraft.server;
 
 import java.util.List;
+import org.bukkit.Bukkit;
 
 public class WorldData {
 
@@ -46,6 +47,10 @@ public class WorldData {
             this.worldUID = (System.nanoTime() << 20) + this.a;
             nbttagcompound.setLong("WorldUID", this.worldUID);
         }
+
+        if (Bukkit.getServer().getWorld(worldUID) != null) {
+            throw new IllegalStateException("World '" + name + "' contains a conflicting UID with existing world. Please edit its level.dat and remove WorldUID, or delete its level.dat (and lose its seed/etc).");
+        }
         // CraftBukkit end
     }