We're all driving around in cars that owe a fundamental design to Henry Ford. But if you disassemble your car, strike molds off of all the parts, and start selling copies, you will find out very quickly (from lawyers) that your car has unique (in a legal and a patent sense) parts.
That doesn't mean you can't go to school and study cars and learn how to design, build, or maintain a car. You will learn this skill by (gasp!) learning about existing cars. And you will learn that at the end of the day, there are only so many ways to design an oil pan.
Dittos for computer software and Operating Systems.
Computers do very few things when you boil it down. They take input, process it, and generate output.
Because Operating Systems often manage identical key tasks, they are going to have design similarities. Sometimes these similarities are known as "standards". Can you say that, boys and girls?
Unix inspired five or six independant clones prior to Linux. These clones are all based off of an open, public standard - Posix. While these Operating systems are similar because they follow a standard, that doesn't make any of these programmers into thieves.
Linus wrote his own code. He also followed stardards. Whether you feel like he's an inventor is your own choice. But if you want to suggest he didn't invent Linux, I'd like for you to consider this:
Henry Ford simply added an engine to a varient of a horse carriage, yet he is considered the "inventor" of the automobile.