Get all values of Enum with specific integer value c#

Well, firstly I would strongly recommend that you don't do this.

Having multiple names for the same value is a really bad idea, IMO. However, you can do it with reflection:

using System;
using System.Collections.Generic;
using System.Linq;

enum Color
{
    Red = 1,
    Green = 1,
    Blue = 1,
    Yellow = 2,
    Cyan = 2,
    Purple = 2
}

class Test
{
    static void Main()
    {
        foreach (var name in GetColorNames(1))
        {
            Console.WriteLine(name);
        }
    }

    static IEnumerable<string> GetColorNames(int value)
    {
        return Enum.GetNames(typeof(Color))
                   .Where(name => (int) Enum.Parse(typeof(Color), name) == value);
    }
}

Personally I would have separate values in the enum instead, and then have a Lookup<int, Color> or something like that. Aside from anything else, it would be very confusing to have something like:

Color color = Color.Blue;

... and then see Red in the debugger or other diagnostics...