An extension to ICollection to provide a .AddIfNotExists() method to remove the need for constant checking.

AddIfNotExists.cs

/// <summary>
/// Adds an item to the list only if it doesn't already exist there.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="coll"></param>
/// <param name="item"></param>
public static void AddIfNotExists<T>(this ICollection<T> coll, T item) {
    if (!coll.Contains(item))
    {
        coll.Add(item);
    }
}