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Alternative Solutions:
1. Open the constituent record
2. Go to Relationships tab and open the relationship of issue
3. On the General 1 tab, mark 'Spouse' checkbox
4. Click Save
5. On the General 2 tab, unmark 'Automatically soft credit this individual'
6. Click Save
7. Go back to the General 1 tab, and unmark 'Spouse' checkbox again
8. Save and close

OR

1. Add the new gift on the constituent record
2. Manually remove the soft credit recipient
3. Save and close the gift