The ghost effect
first of all shoot a scene empty then shoot the scene with the ghost actor, making sure the camara is still on a tripod so that the background can be still, Import both takes of the shot onto premiere, Place the empty shot on the first video track and the ghost shot on the next track, Set the opacity of the ghost shot to around 30%, then you can also add a spooky ghost sound effect to make a good result.
Duplicate a person
First you need to shoot two versions of the scene, each with the actor in a different position. Make half of one of the shots transparent then put them onto premiere but make sure you put them on seperatly so that the top one says video 1 and the bottom one says video two then you can join them together making sure one is a green screen.
Green screen is good because then you can make the background any colour you would like.