--- a/net/minecraft/world/level/entity/EntityLookup.java
+++ b/net/minecraft/world/level/entity/EntityLookup.java
@@ -33,6 +_,14 @@
         UUID uuid = entity.getUUID();
         if (this.byUuid.containsKey(uuid)) {
             LOGGER.warn("Duplicate entity UUID {}: {}", uuid, entity);
+            // Paper start - extra debug info
+            if (entity instanceof net.minecraft.world.entity.Entity) {
+                final T old = this.byUuid.get(entity.getUUID());
+                if (old instanceof net.minecraft.world.entity.Entity oldCast && oldCast.getId() != entity.getId() && oldCast.valid) {
+                    LOGGER.error("Overwrote an existing entity {} with {}", oldCast, entity);
+                }
+            }
+            // Paper end - extra debug info
         } else {
             this.byUuid.put(uuid, entity);
             this.byId.put(entity.getId(), entity);
