Leads are able to be converted multiple times
|Knowledge Article Number||000232519|
|Description||If you're able to convert a single lead multiple times or if you're seeing that view of converted leads has changed, it's because of 2 user permissions.
We'll go over how to modify those permissions to correct the issue.
|Resolution||This is an intended change that was made in the Spring '16 release. Refer to Salesforce Spring ’16 Release Notes on "Leads: Update Details for Converted Leads in Salesforce Classic"
There are two permission that you would want to make sure are not checked to prevent this from happening:
Note: The 2 user permissions mentioned above are only available to be modified on custom profiles. You cannot modify these on standard profiles. If you need to grant access to the two user permissions for user(s), you will need to create/update a permission set.