Duplicating items in Bibliography

Hi Rena here,

188BET靠谱吗Zotero is duplicating items in my bibliography and assigning either a or b after the year eg: (2009a) (2009b) for the same book or article.This happened after combining seven separate chapters into one large document for me dissertation.It usually occurs when I add a new citation for an author already cited.
188BET靠谱吗https://s3.amazonaws.com/zotero.org/images/forums/u14754961/nescyn87z6o7l72dkokf.png

I have tried many methods to straighten this out.

Any assistance appreciated!
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