Hi

I am trying to update picklist value for leadsource through metadata api but not sure why getting 'Concurrent modification from setup pages' error. 
Below is my code :
private bool UpdatePicklist(string sfObject, string picklistName, List<string> picklistValues)
        {
            List<string> extraPickList = new List<string>();
            if (_ms == null)
                GetMetaDataService();
            DescribeSObjectResult sr = _sf.describeSObject(sfObject);
            if (sr == null || sr.fields == null || sr.fields.Length == 0)
            {
                _log.Info("Error occured : sf object description is null");
                return false;
            }

            foreach (Field field in sr.fields)
            {
                if (!(field.type == fieldType.picklist && field.name == picklistName))
                    continue;
                foreach (PicklistEntry entry in field.picklistValues)
                {
                    if (!picklistValues.Contains(entry.value))
                    {
                        extraPickList.Add(entry.value);
                    }

                }
                if (extraPickList.Count > 0)
                {
                    extraPickList.AddRange(picklistValues);
                    CustomField co = new CustomField();
                    co.fullName = sfObject + "." + picklistName;
                    co.type = (SfApi.FieldType)Enum.Parse(typeof(SfApi.FieldType), "Picklist");
                    co.picklist = new Picklist();
                    co.picklist.sorted = true;
                    List<PicklistValue> picklistArray = new List<PicklistValue>();
                    foreach (string picklistValue in extraPickList)
                    {
                        PicklistValue pick = new PicklistValue();
                        pick.fullName = picklistValue;
                        // setting default value
                        if (picklistValue == "Other")
                            pick.@default = true;
                        picklistArray.Add(pick);
                    }
                    co.picklist.picklistValues = picklistArray.ToArray();
                    Metadata[] m = new Metadata[] { co };
                    UpdateMetadata um = new UpdateMetadata();
                    um.metadata = m[0];
                    UpdateMetadata[] uma = new UpdateMetadata[] { um };
                    AsyncResult r = _ms.update(uma)[0];
                    //AsyncResult r = _ms.create(m)[0];
                    while (!r.done)
                    {
                        System.Threading.Thread.Sleep(10 * 1000);
                        r = _ms.checkStatus(new string[] { r.id })[0];
                        _log.Info("Still On");
                    }
                    if (r.state == AsyncRequestState.Error)
                    {
                        _log.Info("Error : " + r.statusCode + " " + r.message);
                        return false;
                    }
                    else
                    {
                        _log.Info("Picklist successfully recreated");
                        return true;
                    }
                }
                else
                    return true;
                break;
            }
            return false;
        }

Please let me know the solution for this, tried many things but no success.