Weird Dupe Glitch of 2nd End

I was playing with friends on a private server and i killed the dragon and as soon as I killed the dragon the 2nd end portal (i don't know the name of it) just spammed everywhere! Only one has the small portal in it ! and when you go through the portals are spammed there aswell ... and when near the portals you go invisble and people cant see you and You can't see yourself in F5 Mode