In many cases you receive the same bug report multiple times from different users, which starts to scatter your bugs list with duplicate bug reports.
Having all this bugs grouped together will help you take your actions once; Assigning it, updating its status, and checking all its conversations in one place. Here comes the benefit of using the "Mark as Duplicate" action in your bug details page.

In this article we will be covering the below points:

Usually while checking your new reported bugs, you start to realize that this bug has been reported before, once you want to relate it to the other bug report that got reported before, just click on the "Mark as Duplicate" button that is on the right side of your bug details.

Once clicking on the button, you can start searching for the relevant duplicate bug.
To find the relevant bug (the one that will become the parent) you can search by the bug description or user email.

Once you mark your bug as a duplicate with another one, your bug will be listed as a child to the parent bug. You will find in your child bug a blue bar at the top of your child bug mentioning "This report is marked as duplicate bug." Clicking on "View Original" will direct you to the original bug report (The parent). 

The parent bug report has all the bugs linked to it as a child. It is the master bug for all the duplicates. The assignee, status, and priority should always match with the parent. Which means, when any of these are changed in the parent bug, all the children should follow the change.
When marking a child bug as duplicate with a parent bug report that doesn't have the same status, assignee or priority, the child should automatically adopt the parent's values. 

You can see all the linked bugs to it by clicking on "# Duplicate Bug" button. It will open a drawer listing all the bugs linked to it as duplicates.

If a child report has a new reply, you will see it in the parent bug. A red dot appears on the # Duplicate Bugs button and you will be able to directly reply from there.

When it comes to the bug list, you might be thinking which bugs are listed. You can think of it as if you have three states of a bug report:

The bugs list contains only state 1 and 3. As for state 2, you can access it from the parent bugs.

Parent bugs will have "# Duplicate bugs" label in the list. 

In case you have multiple bug reports that you want to mark as duplicate at once, you can select them from the bugs list and click on ... then Mark as Duplicate.
​