Domain and Sub domain content connection and using policy with SEO point of view

I am going to make a website for digital marketing for 30 cities in my country. I have decided to make 30 subdomains for each city. I am thinking if I will make a subdomain then I have more chances that I can come easily on the first page of Google. Now my question is:

• Can I duplicate the content on subdomains?
• How much duplication is allowed?
• How to avoid duplication if it's not allowed?
• What is the shortest way to achieve my goal by keeping the SEO in mind?
• Have you ever done this work?
• What is the policy of google if we working on the subdomains?
• Any other suggestion to achieve this goal?