user80233: How can a coinbase transaction be created and what would happen if there was more than one coinbase transaction in a block?

user38482: The current bitcoin system does not allow to create a block with two coinbase transactions. 
The rules are described here. 
So a player against the system would need to craft such a block manually, which would get rejected, and eventually the node would get banned.
Hint: as his is a basic question of how bitcoin works, I recommend to read the technical articles on bitcoin.org, as well as the article on protocol specification in the link. And on top Andreas‘ book „Mastering Bitcoin“ gives a deep insight into the bitcoin world. It is also online available, in many languages. Highly recommended!

80233: <p>How can a coinbase transaction be created and what would happen if there was more than one coinbase transaction in a block?</p>
 38482: <p>The current bitcoin system does not allow to create a block with two coinbase transactions. 
The rules are <a href="https://en.bitcoin.it/wiki/Protocol_rules#.22block.22_messages" rel="nofollow noreferrer">described here</a>. 
So a player against the system would need to craft such a block manually, which would get rejected, and eventually the node would get banned.</p>

<p>Hint: as his is a basic question of how bitcoin works, I recommend to read the technical articles on bitcoin.org, as well as the article on protocol specification in the link. And on top Andreas‘ book „Mastering Bitcoin“ gives a deep insight into the bitcoin world. It is also online available, in many languages. Highly recommended!</p>
