Find all indexes of substring, ignore duplicates

            //List<int> indexes = "fooStringfooBar".AllIndexesOf("foo");
            //List<int> indexes2 = "fooStringfoofooBar".AllIndexesOf("foo");
            //List<int> indexes3 = "oooStringoooBar".AllIndexesOf("ooo");
            //List<int> indexes4 = "oooStringooooooBar".AllIndexesOf("ooo");
            //List<int> indexes5 = "ooooStringooooooBar".AllIndexesOf("ooo");
public static List<int> AllIndexesOf(this string str,char c, int allowedNum)
        {
            var value = new String(c, allowedNum);
            if (String.IsNullOrEmpty(value))
                throw new ArgumentException("the string to find may not be empty", "value");
            List<int> indexes = new List<int>();
            for (int index = 0; ; index += value.Length)
            {
                index = str.IndexOf(value, index);
                
                if (index == -1)
                    return indexes;
                if (index > 0 && str[index-1] == value[0])
                {
                    continue;
                }
                indexes.Add(index);
            }
        }