There is a duplication checker available in your reference list. You can click a button and get a list of the references that are duplicated in a new tab. Access this feature from the 'Add references' menu.
If the list is empty, it means we did not find any.
This is not a very advanced deduplication tool, like you can find in systematic review software, it is more a help for you not to import the same reference twice.
We use this feature to alert you also when you upload new references via PubMed.