Не красиво, но работает для вашего образца: -)
private static IEnumerable<string> GetUniqueWords(string phrase, string word, int amount)
{
//Clean up the string and get the words
string[] words = Regex.Split(phrase.Replace(".","").Replace(",",""),@"\s+");
//Find the first occurrence of the desired word (spec allows)
int index = Array.IndexOf(words,word);
//We don't wrap around the edges if the word is the first,
//we won't look before if last, we won't look after, again, spec allows
int min = (index - amount < 0) ? 0 : index - amount;
int max = (index + amount > words.Count() - 1) ? words.Count() - 1 : index + amount;
//Add all the words to a list except the supplied one
List<string> rv = new List<string>();
for (int i = min; i <= max; i++)
{
if (i == index) continue;
rv.Add(words[i]);
}
//Unique-ify the resulting list
return rv.Distinct();
}
}