using System;
using System.Text.RegularExpressions;
 
public class Example
{
    public static void Main()
    {
        string pattern = @"((?:\A|,)""|\G(?!\A))([^""]*)("")(?!,|\Z)";
        string substitution = @"$1$2$3$3";
        string input = @"something not qualified,""12"" x 12"" something qualified, becuase it has a comma"",this one is not qualified and needs no fixing a 12"" x 12""";
 
        Regex regex = new Regex(pattern);
        Console.WriteLine(regex.Replace(input, substitution));
    }
}

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