Every entity in the world has a unique identifier. When the server loads a chunk and finds an entity whose identifier is already in use, it says so — and if it says so hundreds of times a minute, something copied part of your world.
What you are seeing
- Repeated
Keeping entity ... that already exists with UUIDlines. - The console scrolling too fast to read, with the same message.
- An area where an entity seems to duplicate or flicker.
- The log file growing by hundreds of megabytes a day.
Why it happens
Two entities ended up with the same identifier. That happens when chunks are copied — pasting a schematic containing entities, copying region files between worlds, restoring part of a backup over a live world — and occasionally from a mod that creates entities incorrectly. The server handles it by keeping one and complaining, so it is a symptom rather than a crash.
What to do
- Find where it is happening
The message includes coordinates. If they are all in one area, that area is the one that was copied.
- Check what you did recently
A schematic paste, a world merge, a partial restore. This warning almost always dates from a specific action rather than appearing on its own.
- Remove the duplicated entities
In game with a targeted kill command in the affected area if they are ordinary mobs; with an offline region editor if they are item frames or armour stands you want to keep some of.
- Stop it flooding the log while you work
Hundreds of megabytes of identical lines makes every other problem harder to find. Deal with the cause rather than filtering the message.
Item frames, armour stands, paintings and boats are entities. A broad /kill @e in a decorated area removes a great deal of somebody's work in one command. Target the specific type, and take a backup first.
Most editing tools have an option to exclude entities from a copy. Using it means you paste the build without the item frames that later become duplicates — and you can place those by hand afterwards.
The message stops appearing after a restart, and the log file stops growing at an unusual rate.