If you want to update multiple citations at the same time (eg. adding retrieval date or label), the best time to do it is when you have just imported them (in the 'Imported References' window) or in a Group.
To edit a field in multiple citations:
As your EndNote Library grows and has a lot of references from a variety of sources, you may find some references are duplicated.
Note - You must remove duplicates if you intend to use 'Cite While You Write', to prevent errors in your reference lists.
To search a complete EndNote Library for duplicates as below:
Note - If you select to Keep this record, the other record will be sent to Trash.
Automatically discarding duplicates
To set EndNote preferences to automatically eliminate duplicate references retrieved from an online search: