What happens when a transaction is included in more than one block? What happens when two miners include same or set of transaction in their block, mine it and then broadcast it at same time? Can the transaction be included in two separate block like this OR a fork happens?
If both blocks are broadcast at the same time, and we assume both miners are equally connected to the network, a fork may occur.

Since the default mining policy is mining on the top of the longest chain, or the first one heard in case of a fork, the network should mine in two different blocks until a new one is found, moment when all should go back to "normal".