I do have a lot of duplicate items for myself. My personal collection is a lot bigger than my kit. It's way too much to fit in a traincase anyway. A few products (ones that aren't used too often) do double duty, and those are sanatized before I use them on someone else, and afterwards before I use them again.
Here's what I do to avoid contamination-
Clean brushes between every use ( I keep a seperate set of brushes for my personal use)
Pressed Powder Products -Spritzed with alcohol solution after every use, then top layer is swiped off with clean cotton ball
Loose Powder Products - seperated into small sample containers, I take out some for myself out of the new container when I get the product, and then seperate as needed to work with (I generally give the client anything left over for touchups if needed)
Creme and liquid products- I scrape out or pour a small amount of product onto a plastic palette I use when working to ensure no cross contamination
Mascara- new disposable wand for every dip, never shared between myself and others, replaced frequently
lip product- spray with sanitizer and wipe off the top layer after every use, or for glosses, new disposable wand for every dip and work off product with lip brush
I don't work for MAC, so I can't tell you accurently what their sanitation procedures are. These are just what I do personally.