You're code looks correct, converting multiple leads to the same account should work just fine.
Most likely your leadsToConvert list has the same lead more than once. You can quickly validate this by using a map to prevent duplicates.
List<Database.LeadConvert> leadConverts = new List<Database.LeadConvert>();
Map<Id, Lead> leadMap = new Map<Id, Lead>(leadsToConvert);
for(Lead myLead : leadMap.values()){