[SalesForce] Got DUPLICATE_VALUE exception with value : and record id: in a batch

I got the following exception on a batch job:

System.DmlException: Insert failed. First exception on row 0; first error: DUPLICATE_VALUE, duplicate value found: <unknown> duplicates value on record with id: <unknown>

It fails SOMETIMES when inserting AccountShare objects.

    // Delete existing shares for these accts/users
    Set<ID> acctIDs = new Set<ID>(salesRelColl.getAccountIDs());
    Set<ID> userIDs = new Set<ID>(salesRelColl.getUserIDs());
    EntitlementManager.deleteSharePermissionsForAccounts(acctIDs, userIDs);
    String relatedListsAccessLevel = EntitlementManager.ACCESS_LEVEL_EDIT;
    Map<ID,Set<ID>> acctUserMapForCptyShares = new Map<ID,Set<ID>>();

    List<AccountShare> shares = new List<AccountShare>();
    List<AccountShare> sharesOwner = new List<AccountShare>();

    List<Id> accountIDs = new List<Id>();
    Map<Id, Account> accountOwnerMap = new Map<Id,Account>();

    for ( Sales_Relationship__c salesRel : salesRelColl.getList() )
    {
      accountIDs.add(salesRel.Account__c);
    }

    AccountSelector accountSel = new AccountSelector();
    AccountCollection accountCol = accountSel.whereIDIn(accountIDs).execute();

    for(Account acct : accountCol.getList())
    {
      accountOwnerMap.put(acct.Id,acct);
    }

    for ( Sales_Relationship__c salesRel : salesRelColl.getList() )
    {

      ID userID = salesRel.Employee__r.User__c;
      if ( userID == null )
      {
        continue;
      }
      // exclude any non-current
      if ( !salesRel.Current__c )
      {
        continue;
      }

      if (accountOwnerMap.get(salesRel.Account__c) != null)
      {
        if (accountOwnerMap.get(salesRel.Account__c).OwnerId != userID)
        {
          ID acctID = salesRel.Account__c;
          String groupType = ROW_CAUSE_SALESPERSON;
          ID parentID = acctID;
          ID userOrGroupID = userID;
          String accessLevel = EntitlementManager.ACCESS_LEVEL_EDIT;
          String rowCause = groupType;

          AccountShare share = EntitlementManager.buildShareForAccount(parentID,
                    userOrGroupID,accessLevel,
                    relatedListsAccessLevel,
                    rowCause,groupType);

          shares.add(share);

          if ( !acctUserMapForCptyShares.containsKey(acctID) )
          {
            acctUserMapForCptyShares.put(acctID,new Set<ID>());
          }
          acctUserMapForCptyShares.get(acctID).add(userID);
        }
        else
        {
          AccountShare share = EntitlementManager.buildShareForAccount(salesRel.Account__c,userID,EntitlementManager.ACCESS_LEVEL_EDIT,
                    relatedListsAccessLevel,
                    ROW_CAUSE_SALESPERSON,ROW_CAUSE_SALESPERSON);
          sharesOwner.add(share);
        }
      }
    }

    if ( shares.size() > 0 )
    {
      // FAILS HERE
      insert shares;

    }

The fact that is something that does not happen all the time had made this bug not easy to reproduce.