* Prevent adding duplicated chapters into the db
it's possible that the source returns a list containing chapters with the same url
once such duplicated chapters have been added, they aren't being removed anymore as long as there is
a chapter with the same url in the fetched chapter list, even if the duplicated chapter itself
does not exist anymore on the source
* Drop duplicated chapters from database table
* Add unique constraint to chapter table
This is to completely prevent duplicated chapters from being added to the database.
Since once a duplicated chapter has been added to the database, it does not get removed anymore as long as a chapter with the same url is included in the requested source chapter list
schroda
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