When a player goes into a End Portal in Overworld side, if there is a End Portal in the end with the same coordinates as the former, there is a little chance that the player will be sent back to the spawnpoint of the overworld immediately. It seems that an extra teleporting happen in this process. (It is weird that this phenomenon doesn't happen when player go into a End Portal in The End)
p.s. If the player is sent to the End Portal by using /tp, the chance will increase(1 in 5 perhaps).

To reproduce it:
prepare 2 End Portals with same coordinates in both Overworld and The End, for example, (-455,25,-673). Then use /tp @s -455 25 -673 in Overworld for several times, you may find that you see the "loading terrain" screen twice in a row and yourself in the Overworld spawn point
(you may see the gif file in the attachment for a preview)