When attempting to copy an item, ShareGate Migrate displays the following warning:

Unable to determine if this item already exists due to the list view threshold. The item will be added.

SharePoint's list view threshold is set to a default of 5000 list items. You will receive the above warning if you are copying items to a list that exceeds the list view threshold.

SharePoint returns an error to queries when the number of items in a list exceeds the list view threshold. This means that when ShareGate Migrate queries the destination to verify if a list item already exists, ShareGate Migrate will always receive an error from SharePoint.

Since ShareGate Migrate cannot determine if a list item already exists at the destination, ShareGate Migrate will create the list items as new items. This means that any items that already existed will not be updated but will instead be duplicated.

It is best practice is to make sure that the destination list respects the list view threshold.

Create folders in the destination list and organize your items by folder. Limit the number of list items in each folder to less than 5000 items.

As SharePoint's list view threshold only applies to the number of items in a given scope, splitting the items into folders will resolve the issue.