Overview

When using a spawner minecart with a spider as the spawn potential, it has a chance to spawn a spider jockey, when this happens the game will crash with the error:
"The game crashed whilst exception ticking world
Error: java.lang.IllegalStateException: Entity is already tracked!"
It only effects spawner minecarts not normal spawers

From testing it looks to only be spider jockeys causing it not any other jockeys

Recreate
Summon the following spawner in:

summon spawner_minecart ~ ~ ~ {SpawnCount:20,MinSpawnDelay:50,MaxSpawnDelay:100,MaxNearbyEntities:100,RequiredPlayerRange:16,SpawnData:{id:"minecraft:spider"}}

If you summon a few in a give it a minute or 2 the game will crash, when your relog you will see a spider jockey has just been spawned in.