Duplicate account IDs when using Database.convertLead()?

Question

Does Database.convertLead() allow conversion of multiple leads to the same account at once? That seems like it should be no problem, but I'm getting strange errors in my unit testing.

I wrote a lead trigger to convert all new leads. In my unit test, I create two leads and put them in a list. The leads have different names and email addresses but the same Account ID. (This is a non-profit and they place all their contacts in a single "bucket" account.)

When I insert the lead list, the leads are converted, but I also find these errors in the debug log:

08:09:50.608 (20608468000)|FATAL_ERROR|System.ListException: Duplicate id in list: 001J000001EZwn6IAD

(System Code)

08:09:50.608 (20608490000)|FATAL_ERROR|System.ListException: Duplicate id in list: 001J000001EZwn6IAD

(System Code)

I don't understand why these fatal errors are occurring on the account ID and yet the leads are converting. I know there are no duplicate leads in the LeadConvert list because I'm using map.Values() in the for loop to populate the LeadConvert list, and also I have examined the LeadConvert list before it converts.

@James - Actually I found your post earlier but I didn't want to believe that convertLead() couldn't convert multiple leads with the same account. What's even more confusing is that the leads get converted despite the error. It doesn't make sense.

@Jason - Here's my trigger handler class. I'm passing Trigger.new and Trigger.newMap to it.

public class LeadAfterInsertHandler {

    public static void process(Lead[] newList, Map<Id, Lead> newMap) {
        String msg = '';
        Set<String> emailSet = new Set<String>();
        Map<String, Contact> conMap = new Map<String, Contact>();
        Contact[] consToUpdate = new List<Contact>();

        //New contacts are linked to an account named Individual (a bucket).
        //The ID is stored in Custom Settings.      
        Lead_Conversion_Settings__c settings = Lead_Conversion_Settings__c.getInstance();

        //Get email addresses.
        for(Lead l : newList) {
            emailSet.add(l.Email);
        }

        //Get existing contacts.        
        for(Contact con : [select Email from Contact where email in :emailSet]) {
            conMap.put(con.Email, con);
        }

        //Prep the leadConvert object.
        Database.LeadConvert[] lcList = new List<Database.LeadConvert>();
        for(Lead l : newList) {
            Database.LeadConvert lc = new Database.LeadConvert();
            lc.setLeadId(l.Id);
            lc.setConvertedStatus('Closed - Converted');
            lc.setDoNotCreateOpportunity(true);
            lc.setAccountId(settings.Individual_Account_ID__c);         
            //Contact Id if it already exists.
            if(conMap.keySet().contains(l.Email)) {
                lc.setContactId(conMap.get(l.Email).Id);
            }
            lcList.add(lc);
        }

        //Convert. 
        if(!lcList.isEmpty()) {
            Database.LeadConvertResult[] lcrList = Database.convertLead(lcList, false);
            for(Database.LeadConvertResult lcr : lcrList) {
                //Capture failed conversions for the email message.
                if(!lcr.isSuccess()) {
                    msg = msg + newMap.get(lcr.getLeadId()).FirstName + ' ' + newMap.get(lcr.getLeadId()).LastName + ' conversion error: '
                     + lcr.getErrors()[0].getMessage() + '\n';
                }
            }
        }

        //Send email with failed conversions.
        if(msg != '') {
            Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
            String[] toAddresses = new String[] { settings.Error_Message_Recipient__c };
            mail.setToAddresses(toAddresses);
            mail.setSubject('Lead conversion errors');
            mail.setPlainTextBody(msg);
            Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });           
        }
    }
}

Thanks

David