extra-unique-identifiers

Dictionary<string, bool> usedGuids = new Dictionary<string, bool>();

private string CreateUniqueGuid()
{
    string newGuid = Guid.NewGuid().ToString();

    if (usedGuids.ContainsKey(newGuid))
    {
       throw new Exception();
    }

    usedGuids.Add(newGuid, true);

    return newGuid;
}