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If you run your searches across multiple databases it likely that you will get the same citation in your collection multiple times.
To remove duplicate citations there are two main options:
Order your citations by author and manually review and delete individual citations, or
Use RefWorks de-dupe tool. You can find this under Tools and then clicking Find duplicates (first image). You can choose Exact Match or Close Match. You will then see a list of pre-selected citations that you can then delete (second image).