@copyrite - it isn't related to this plugin in that the answer is no different to if you added an empty book via the Add menu. Whether a duplicate book gets created or the format gets merged comes down to (a) how you import the book and (b) what your add book settings are.
If you do as theducks says and search for the book first and then drag/drop onto the book details panel or open the edit metadata dialog and drag/drop onto the formats area then it will 100% merge.
If you drag/drop onto the main window or add using the Add dropdown, then it firstly depends on your settings for reading metadata/filename regular expression etc. That will determine whether the book is evn identified as "possibly" a duplicate.
Assuming it does match, then it comes down to whether you have ticked the Automerge box in Preferences -> Adding books and which option you chose. See the help/other forum threads for more info on that.
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