Summary
You want to merge duplicate deals into a single deal instead of deleting or marking one of the duplicates as “lost.”
Resolution
Currently, there is no option to merge duplicate deals automatically. You can manually add details from the duplicate deal(s) to the original deal, and then delete the duplicate(s).
However, if you merge duplicate contacts, the deals associated with the contacts will also merge. For example, if Contact A is merged to Contact B, the deals associated to Contact A will be transferred to Contact B.
Cause
There are duplicate Deals for the same contact.