red
red
Grey
Grey
red
blue
blue
Green
White
Grey

red    Count=2  index=0 (contain the duplicate cluster start index)
Grey   Count=2  index=2
red    Count=1  index=4
blue   Count=2  index=5
Green  Count=1  index=6
White  Count=1  index=7
Grey   Count=1  index=8

var result = from p in a //a is the list
                         group p by p into g
                         select new { value=g.Key,count=g.Count()};