One possible cause of this error is that one of the constituent's has a link to the INTERACTIONADDITIONALFUNDRAISER table where the fundraiser is the other constituent in the merge. This is a problem because the merge operation updates the source's INTERACTION table's records to be the target's records so a constraint on that table checks the INTERACTIONADDITIONALFUNDRAISER to ensure that the fundraiser on the first table is not duplicated by the second table.

The constraint prevents a scenario where you have an Interaction where the Fundraiser constituent is also list as an additional fundraiser on the Interaction.

To resolve this issue, delete the INTERACTIONADDITIONALFUNDRAISER table entry.