Particles generally double up, you need to go to your particle folder and delete any double files.
Make sure you're setting coordinates for these portals, otherwise people won't be able to find them, because they'll be on the edge of the map and unreachable. Use the XYZ's, and match it up with the numbers in the right hand corner in map maker, the last number is your yaw, the direction you're facing. That's the spot and direction upon which players would enter your map.