If you report two bugs whose reproducible steps may be different and the end result may be different but the cause of the defect is same then it would be considered as duplicate bug.
Assume that there are two testing engineerers testing the application but initial flow is same or senarios. By that time the defect found by the test engineer 1 is same as test engineer 2 is called as duplicate Bug.
If any of the tester logged the same issue twice in the same release then it will considered as a duplicatite bug this will occured in below scenarios.
-> If a build was tested by 3 members then there may be a chance to log the same issue by more than one tester
-> If a build contains a lot many bugs then there may be a change to log the same issue twice
-> We need to test the build in 3 servers ( Our server client test server and Product) there may be a chance to log the same issue in any of the 2 servers ( if status of issue was changed to Deffered)
Same bug is reported twice thrice or more by same Tester or different Tester in the Bug Tracking tools which represent the same test steps or different test steps but the bug cause is same is called Duplicate Bugs.
