using System;

using System.Collections.Generic;

using System.Linq;

​

ILookup<string, string> AuthorList = new[]

{

    KeyValuePair.Create("aaa", "гистидин"),

    KeyValuePair.Create("bbb", "фенилаланин"),

    KeyValuePair.Create("aaa", "кокаин"),

    KeyValuePair.Create("ccc", "глицин"),

    KeyValuePair.Create("ddd", "глицин"),

    KeyValuePair.Create("aaa", "атропин")

}.ToLookup(kvp => kvp.Key, kvp => kvp.Value);

​

var results = AuthorList["aaa"];

foreach (var result in results)

    Console.WriteLine(result);