Do you mean you want the indexes of each occurence, then perhaps this?
string[] lots = new string[] { "one", "two", "three", "six", "six", "two", "four", "five", "six" };
var x = lots.Select((s, i) => new { Index = i, str = s })
.GroupBy(i => i.str)
.Select(g => new {Str = g.Key,
Indexes = g.ToList() });