First of all you will need some fancy camera work. To get a good finished multiplicity photo it will be important to take the photos in relatively quick succesion, especially if shooting a photo sequence outdoors. This will ensure that the lighting and shadow in the photos will be coming from the same direction across all shots.
You will need to set up a camera which will be able to take in the full background of all the photos you wish to take. Here you should test the lighting and get all the camera settings correct.
Once you are ready to take the initial picture do not change any of the settings until you have taken all the images in sequence.
The first image should just be the background.
Then take the exact same photo for all the different shots.
After exporting the photos onto your computer, open up the plain background in photoshop.
Open up one of the sequences and using the arrow tool from the toobar click on the canvas of the new photo and drag this image until your mouse pointer is posistioned over the background document. (To do this you should have your document in standard screen mode.)
The photos should be exactly the same size so simply line them up. To make sure this is exact, use the transform option. With the new photo selected in the layers palette Ctrl T or Apple T to display the transorm controls, select the top left point in the transform application tool bar and type 0, 0 into the X & Y fields. The image should move to perfect alignment. Hit the return or enter key to deselect the controls.
Repeat step 5 with the other photos you have until each photo sequence in displayed on top of one another on top of the background layer.
Save the document at this point so you can go back to this point if you need to.
Toggle the eye tool in the layers palette to hide the different layers until there is just the first photo in the sequence left on. Turn off the background also.
Select the eraser tool and right click on the canvas to set the eraser size and hardness. If you mouse over the canvas you will see a circular shape to show how big the brush size is currently set to. (If this is not showing, and only showing a cross hair, it may be that you have caps lock on, press caps lock to revert to the circular shape) Set the brush to a suitable size in comparison to your photo.
For the hardness I would recommend around 60%
Press return or enter to exit the brush size panel
Using the eraser delete all parts of the photo except for the object you want to use for that sequence. Leave a border of blended background around the object.
Turn on the background and check that the edge of the top layer is seemless with the background. Continue to erase more of the edge with a soft eraser and turn down the opacity of the eraser too if needs be.
Repeat this process with the other object layers. Switch on all the object layers at the end to see the full effect.
Save as a psd file so you can make future alterations accross all layers if needs be.
Flaten and save as a jpeg.
Multiplicity examples, some of my favourites found on flickr: