Duplicate prevention upon manual entry in Salesforce shows the actual potential duplicates in the native entry page upon their entry or update. It prevents the saving of a duplicate record and gives the possibility to bypass this prevention by the Disable Duplicate Check checkbox.
Duplicate Check Manual Insert prevention will trigger:
upon the creation of a new record
upon the update of an already existing record
When it's triggered, the feature will show a warning message and a list of the found duplicates.
In Salesforce Classic we can show the potential duplicate records (see example image above) since we can fully integrate into the native entry page of Salesforce. In Lightning, Salesforce has closed the page off and limits what Duplicate Check can show to only a text notification. Third party apps are not allowed to use any code and therefore we can't show the potential duplicate record that is found in details. To see the duplicates found and be able to proceed them further, you need to replace the Salesforce native entry page with the DC Entry page. With the DC Entry page, we are able to show the actual duplicates, not only a text notification.
Learn how to replace the native Salesforce entry page with DC Entry page in this article.
If a new entry record is shown as a duplicate, but you consider it as a fake duplicate, simply allow the creation of the record by disabling the Duplicate entry/update prevention for this record by checking the Disable Duplicate Check checkbox. Learn how to add the "Disable Duplicate Check Checkbox" in this article.
Learn how to add and configure DC Live in Salesforce Lightning here. Learn how to add and configure DC Live in Salesforce Classic here.