While trying to merge two Lead records in Apex, you may encounter unexpected behavior or errors when one or more of the leads are already converted. Specifically, attempts to use a converted Lead as the primary record in the Database.merge() method will fail or be blocked.
Salesforce imposes a platform-level restriction on converted Leads:
On the other hand:
When performing lead merges in Apex:
https://help.salesforce.com/s/articleView?id=sales.leads_merge_considerations.htm&type=5
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