Clients and server are not in sync.

On a multiplayer server, clients are not in sync with the server.

On my point of view, a player is standing on a glass block. on their point of view, they are re-logging. 20 seconds later, they log out for me. 20 seconds earlier, there are two of them on the server. I added a attachment of my friend next to her "clone" while she was lagging.