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

The value provided already exists in another list item.

The error should appear on a given property, for example, Property Title: The value provided already exists in another list item.

The error occurs because the destination column is set to enforce unique values and a duplicate value already exists in the destination list or library. For example, if the column Title enforces unique values and an item already exists in the destination list with the title "Demo", copying a new item with the title "Demo" will cause this error.

If you need to copy the item and it is valid for you to have a duplicate value in the given column, you can change the destination column settings to stop enforcing unique values. This can be done in SharePoint by going in the list or library settings, then clicking on the column and setting Enforce unique values to No.

If you want to overwrite the item that currently exists on the destination instead of creating a duplicate, then there might be a problem with the selected properties. You can change the key properties to only set a key on the column that enforces unique values. The next time you will copy, ShareGate Migrate will consider the source item you tried to copy and the existing destination item as the same and it will ask you whether you want to overwrite it.