Prevent duplicate user in Email

Hi, i have requirement to create email in CRM using C#. 

The following is a piece of code used to get the list of recipients.

for (int iusr = 0; iusr < dtableUserQuality.Rows.Count; iusr++)
{
systemuserid = Guid.Empty;
systemuserid = new Guid(dtableUserQuality.Rows[iusr][0].ToString());
QueryExpression to_query = new QueryExpression("systemuser");
to_query.Criteria = new FilterExpression();
to_query.Criteria.AddCondition("systemuserid", ConditionOperator.Equal, systemuserid);
to_query.ColumnSet = new ColumnSet(true);

EntityCollection to_colect_email = _service.RetrieveMultiple(to_query);


toUserParty.EntityName = "systemuser";

foreach (Entity to_ent_email in to_colect_email.Entities)
{
string fullname = to_ent_email.GetAttributeValue<String>("fullname");



Entity fromParty = new Entity("activityparty");
EntityReference userParty = new EntityReference("systemuser", to_ent_email.Id);

fromParty.Attributes.Add("partyid", userParty);

toUserParty.Entities.Add(fromParty);

}
}

if (toUserParty.Entities.Count > 0)
{
email.Attributes.Add("to", toUserParty);
}

The email created succesfully, however the code allowed duplicate user exist in the recepients list. 

how to prevent users already in the recipient list from being duplicated.

Thanks