var items = new List<string>();
foreach (var item in c.Items.Cast<string>())
{
foreach (var split in item.Split(','))
{
items.Add(split);
}
}
c.Items.Clear();
c.Items.AddRange(items.Distinct().ToArray());
c.Items.Cast<string>().ToList().ForEach(item => item.Split(',').ToList().ForEach(split => items.Add(split)));