using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var collection = new List<Personne>

            {

                new Personne{Nom="sara" ,Acces="skc1 / tsk1"},

                new Personne{Nom="sara" ,Acces="skc1 / tsk2"},

                new Personne{Nom="sara" ,Acces="skc1 / tsk3"},

                new Personne{Nom="sara" ,Acces="skc1 / tsk4"},

                new Personne{Nom="sara" ,Acces="skc1 / tsk5"},

                new Personne{Nom="soufiane", Acces="skc1 /tsk1"},

                new Personne{Nom="soufiane", Acces="skc1 /tsk2"},

                new Personne{Nom="soufiane", Acces="skc1 / tsk3"},

                new Personne{Nom="sawsan", Acces="skc1 /tsk1"},

                new Personne{Nom="sawsan", Acces="skc1 /tsk2"},

            };

​

            var uniqueKeys = collection.Select(x => x.Nom).Distinct();

​

            var collectionWithUniqueKeys = new Dictionary<string, IEnumerable<string>>();

            foreach (var cle in uniqueKeys)

            {

                collectionWithUniqueKeys.Add(cle,collection.Where(x=>x.Nom==cle).Select(x=>x.Acces));

            }

​

    }

    public class Personne

    {

        public string Nom {get; set;}

        public string Acces {get;set;}

    }

}