When running multiplayer server, this sometimes occurs:

[14:35:20] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3989ms behind, skipping 79 tick(s)
[14:35:58] [Server thread/WARN]: Tried to add entity with duplicate UUID 3a3d3602-6551-4c05-a4b3-08a997210999
[14:35:58] [Server thread/WARN]: Tried to add entity with duplicate UUID 5d690dc4-b15b-49ed-9b3f-aeaf6ad2dd4d
[14:36:01] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3468ms behind, skipping 69 tick(s)

and it causes random lag spikes, I'm running the .jar file with 8GB RAM, so it's not a RAM issue, I've had this since 15w31a, with multiple players in a server it's very annoying and sometimes getting kicked from the server.