When attempting to copy a list item, the ShareGate migration tool displays the following error:
This item has not been copied since there are multiple matching items at the destination. Please review your key properties.
When the ShareGate migration tool copies list items, there is no implicit way to know if the item being copied already exists in the destination of your migration, so it uses key Properties to avoid overwriting items.
When the migration tool displays this error, it means that multiple items at the destination of your migration match the source item Key Properties. The ShareGate migration tool can't determine which list item to overwrite at the destination, so it does not copy it.
You can use one of your list columns with unique properties for each item as a key property.
For example, you are copying an item named Meeting 1, created on January 15th, and your destination already has an item called Meeting 1 that you created on February 7th.
If Created is not set as a key property, the migration tool will not know they are different items.
To define your key properties, see adjust your Key Properties.