When attempting to copy a list item, Sharegate 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 Sharegate copies list items, there is no implicit way to know if the item that is being copied already exists in the destination of your migration, so it uses the Key Properties feature to avoid overwriting items.
When Sharegate displays this error, it means that multiple items at the destination of your migration match the source item Key Properties. Sharegate can't determine which list item at the destination must be overwritten so it does not copy it.
As an example, take a migration where you are copying the item "Meeting 1" that was created on January 15th. If your destination also has an item called "Meeting 1" that was created on February 7th, Sharegate will not know they are different items unless "Created" was set as Key Property. By default, Sharegate determines the best key properties for your list to prevent overwrites from occurring. It is always recommended you set key properties yourself to ensure that they are defined to your preference.