With your long list of citation info over the years, there might be chances of duplicated citation info. Reference manager is very handy in dealing with this problem. The following picture is how you do it in Zotero.

Just go to the “Duplicates” tab in your left side pane.  It will detect duplicated entries in you collections or libraries. Then click on the series of duplicates then hit “Merge items” on the right side pane.