I sympathize with the Original Poster, not only because Lenovo screwed up and he ended up paying full price for a service that was subpar, but also because of the duplicate files. Not everyone is as computer savvy as many of the posters here; indeed, that is precisely why the OP hired someone to do the job for him. Yes, storage is cheap, but I still wouldn't want to deal with a computer where all my files were duplicated, depending on how it was done. Imagine keeping track of that! (Now imagine it again, from the perspective of someone who is not computer-savvy.) Imagine twice the files to wade through. Imagine the confusion if you update a file and another lurks elsewhere with the same name. You'd have to rename a file everytime you use it to make it distinct from the other one. Not to mention that if OP then buys a new computer in the future, it will not be so easy to get that transferred without picking up those duplicate files all over again. I agree with smd, a do-over should be fairly trivial if the old computer still exists. And a do-over and a refund is what JJJJohnson deserves.
This was exactly my first thought.
"Imagine twice the files to wade through. Imagine the confusion if you update a file and another lurks elsewhere with the same name. You'd have to rename a file everytime you use it to make it distinct from the other one."
As long as you follow this idea from the beginning, you'll be fine:
When you bring up a file to work on, save it with an A as the first letter of the file name. Create a directory (right click on your desk top, new, folder)* and save (save as) all files you've worked on to that directory. Even if you forget or mush it up, the worked-on file will start with A, no matter which directory it's in.
It seems like yesterday when I was as terrified as you are to do anything with a computer. The words "single executable" would have given me hives. I remember giving a laptop to a friend ... I was talking to her a few months later about how to do something and told her to double-click on whatever. "Double click? What's that?" she responded, I nearly fainted. She had just turned the machine off whenever she was finished ... for months. And it didn't hurt the computer a bit.
You might want to look for a "dummies" book on how to use your computer. I've found them immensely helpful for many subjects over the years. Good luck!
* If you get tired of looking at "new folder" on your icon, right click on the icon, chose rename.