using System;
using System.Linq;
using System.Collections.Generic;
 
public class Test
{
	public static void Main() {
		var dict = new Dictionary<string,List<int>>();
		dict.Insert("x", 1);
		dict.Insert("x", 2);
		dict.Insert("x", 3);
		dict.Insert("y", 4);
		dict.Insert("y", 5);
		foreach (var p in dict) {
			Console.WriteLine("{0} - {1}", p.Key, string.Join(", ", p.Value.ToArray()));
		}
	}
}
 
static class DictExtensions {
	public static void Insert<TKey,TVal>(this IDictionary<TKey,List<TVal>> d, TKey k, TVal v) {
		List<TVal> current;
		if (!d.TryGetValue(k, out current)) {
			d.Add(k, new List<TVal> { v } );
		} else {
			current.Add(v);
		}
	}
}

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