Will I double my money after a bitcoin hard fork? Could anyone please explain to me how this fork will work? 

Examples would be greatly appreciated. The way I'm understanding is, if I have 1 BTC on Coinbase, they will match it. So after the fork I will have my 1 BTC and 1 B2X (Bitcoin 2x). Let's just say I sold my 1 BTC, will I still have my 1 B2X to sell? Does this mean free money?
No, it doesn't mean free money.

When a cryptocurrency forks into 2 separate cryptocurrencies, then the market sets the value for each. 

Much of the value of Bitcoin is based on several things, such as the existing base of businesses that accept it, the efficacy and reliability of the developers maintaining it, and the familiarity by people that use it.

After a fork, that value becomes reduced, as many users or businesses pick one or the other to use. 

New value may be created by new functionality added by the changes that caused the fork, or overall value (ie the sum of both forks) may become reduced due to a loss of trust in the system.

A more reasonable expectation is probably that the overall value, between both forks, is roughly the same as before the fork, but becomes split between two. 

However, given the speculative nature of a sizeable proportion of the market, any small change may trigger large market movements, so it's anyone's guess as to what will actually happen.

After the fork, if you sell your BTC, then yes you will still have your B2X to sell (assuming that the wallet service you're using gives you access to it, or that you run your own wallet).