using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

            List<Settings> processedList = new List<Settings>();

            var jsonExample= "[\r\n{\r\n    \"Domains\": [\r\n      \"domain1.com\"\r\n    ],\r\n    \"Type\": \"imap\",\r\n    \"Hostname\": \"mail.altechauto.com\",\r\n    \"Port\": \"993\",\r\n    \"SocketType\": \"SSL\",\r\n    \"UserName\": \"%EMAILADDRESS%\"\r\n  },\r\n  {\r\n    \"Domains\": [\r\n      \"domain2.com\"\r\n    ],\r\n    \"Type\": \"imap\",\r\n    \"Hostname\": \"mail.altechauto.com\",\r\n    \"Port\": \"993\",\r\n    \"SocketType\": \"SSL\",\r\n    \"UserName\": \"%EMAILADDRESS%\"\r\n  }]";

            var deserealizedClasses = System.Text.Json.JsonSerializer.Deserialize<List<Settings>>(jsonExample);

​

            foreach (var keySetting in deserealizedClasses.GroupBy(settingForGroup => settingForGroup.Hostname))

            {

                var defSettings = keySetting.FirstOrDefault();

                Settings newRow = new Settings()

                {

                    Hostname = keySetting.Key,

                    Domains =  new List<string>(),

                    Port = defSettings?.Port,

                    SocketType = defSettings?.SocketType,

                    Type = defSettings?.Type,

                    UserName = defSettings?.UserName

                };

​

                foreach (var settingRow in keySetting)

                {

                    foreach (var domain in settingRow?.Domains)

                    {

                        if(newRow.Domains.Contains(domain)) continue;

                        newRow.Domains.Add(domain);

                    }

                }

​

                processedList.Add(newRow);

            }

​

            Console.WriteLine(System.Text.Json.JsonSerializer.Serialize(processedList));

    }

}

 public class Settings

    {

        public List<string> Domains { get; set; }

        public string Type { get; set; }

        public string Hostname { get; set; }

        public string Port { get; set; }

        public string SocketType { get; set; }

        public string UserName { get; set; }

    }

​