Most players consider this an act of theft and rudeness since the pirate who first engaged the enemy will not get the same amount of Reputation they would be awarded for defeating the enemy alone. Instead, they are forced to share it with the outside pirate who intervened. This is where the term "Rep Stealing" comes from.
Alternatively, some players consider this to be a helpful gesture. A pirate who has nearly been knocked out may be saved by another pirate running in to help them defeat their powerful foe, allowing them to fight on.
Often, an inexperienced pirate may be unaware that they are doing so and are simply trying to help. However, experienced players take issue with another pirate consistently taking part of their rewards.
One way to prevent Rep Stealing and be polite to other players is to start a crew together.
Joining or creating a Crew effectively nullifies the consequences of fighting an enemy together. By being in a crew, ALL members are rewarded fairly, and get a reward bonus that varies depending on the size of the crew (from 20% up to 50%).
Another way to prevent Rep Stealing is by using a powerful gun -- such as a blunderbuss -- to defeat the enemy with one shot, ensuring full ownership of the rewards. However, be warned that this could also be used by Rep Stealers who have more powerful guns.