Update: This issue has been resolved by Salesforce.
Previously, there was a known Salesforce issue where merging Person Accounts in Nonprofit Cloud (NPC) would fail if both Person Account records had associated Donor Gift Summary records.
As a workaround, users were advised to delete the Donor Gift Summary from the non-master record before performing the dedupe. After the merge, the Donor Gift Summary would then be updated on the master record.
Current Status - 2026
Salesforce has confirmed that this issue has been fixed. Person Accounts in Nonprofit Cloud can now be merged successfully, even when both records have associated Donor Gift Summary records.
The previous workaround is no longer required.
For more information about the Salesforce fix, refer to the official Salesforce Known Issue: ClickHere.