Think you might have loaded a resource more than once? Let Mendeley help you find it and merge the records.
Mendeley usually syncs Desktop and Online components when you exit the account, but in cases where this isn't automatic, you can also manually sync.
Sometimes a resource is missing details (authors, dates, abstracts, etc.). Mendeley has Needs Review to help manage these. Click on Needs Review to edit & manually add the missing details.