Unable to determine if this item already exists due to the list view threshold

Symptoms

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

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

Error code

Details

Note: If you are copying the items for the first time, you can ignore this warning. All the items will be copied correctly.

This warning only indicates that there is a problem if you are copying items that already exist at the destination (in order to update them). The warning happens when copying items to a list that contains more items than the list view threshold, which is set to 5000 by default. The reason for this warning is that Sharegate cannot query the destination list to verify if the item being copied already exists, because SharePoint will return an error to queries to a list with more items than the threshold. So Sharegate will always create the item being copied, which might result in duplicates if the item already existed in the destination list (the item will be created as a new item instead of updating the existing item).

Solution

Make sure that the destination list respects the list view threshold. The best way to do this is to create folders in the destination list to order the items by folder that all have less items inside them. The list view threshold only applies to the number of items in a given scope, so splitting the items into folders will resolve the issue.