When attempting to run an incremental copy on lists with items that contain the same value for "Title" and "Created" in the destination, you see this message:
This item has been copied as a new item since there is multiple matching items at the destination.
Title and Created are key properties used to determine unique items, so when these columns have the same values for multiple items, Sharegate won't be able to determine which ones to replace, and will copy a new item instead.
To resolve this issue:
In order for Sharegate to see your unique items correctly, you must remove the duplicates from that migration job, set new key properties to columns that have unique values, and re-do the migration of your items using Copy SharePoint Content.