It is really disappointing that this bug is still present in recent versions of endnote. in 2013 and 2014 this “feature” was reported –
Basically, If the author is the same and the book title is the same (the secondary title is not considered in footnotes when determinining whether something is a duplicate). adding an extraneous character to the book title in one of the records can sometimes “trick” endnote into thinking they are different.