When attempting to copy an item, Sharegate displays the following error:
There are multiple destination items that correspond to the given key values for the lookup item '1'. The first one was assigned, but it might not be the one that was expected.
The error occurs when the given field points to an ID column and Sharegate finds multiple corresponding items at the destination. Since the ID is not preserved at the destination, Sharegate needs to map the source item to a destination item based on the Unique Key Properties. These properties are usually enough to ensure only one match at the destination, but it is possible that they are not sufficient.
The error serves to prevent unnecessary clutter at the destination. You should look at your Key Properties to verify if you actually do want duplicates at the destination. If yes, you can always select Copy and Keep Both in your General copy options under Advanced Copy.