Have you tried searching for related domain names? Ex: You go to namecheap.com and type in domain names to see if they are taken or not. Also, you could type in the domain name of the niche you want to target. Ex: bluewidget.com bluewidget.net and so on... Might give you clue if you will step on something dangerous so to speak...
Why not start a business you're passionate in? Starting a business you don't really care about won't last long. Find something you love to do and make a business doing it. You'll see greater rewards that way
What common business do you mean? And if you are looking for ideas from us, you will get sooooo much that it's overwhelming. So, just follow what you love That will bring you to the best startup business for you. Good luck!
Your question is a little vague, but here's my take.... What does it matter? Look at the sub sandwich industry. Subway had the market, then along came numerous others; Larry's Giant Subs, Firehouse Subs, Jimmy Johns, Jersey Mike's and more. Same can be said with the pizza industry and numerous other fast food companies. Just because there's already a company in business with a similar or the same idea, which you thought of but didn't realize someone else was already doing it, doesn't mean your's can't exist. Go for it. Who knows, your business with the right marketing twist might even be more successful!