Георгий Кузнецов

document.addEventListener("DOMContentLoaded", function(event) {
  var img = new Image();
  img.src = 'images/background.bmp';

  document.body.appendChild(img);
});

using System;
using System.Collections.Generic;
using System.Text.Json;

var lines = new[]
{
    "1:a",
    "1:b",
    "2:c",
    "3:d",
    "3:x",
    "biba:boba:buba",
};
var result = ParseContacts(lines);
Console.WriteLine(JsonSerializer.Serialize(result, new JsonSerializerOptions(JsonSerializerDefaults.General)
{
    WriteIndented = true
}));

static Dictionary<string, List<string>> ParseContacts(IEnumerable<string> lines)
{
    var dict = new Dictionary<string, List<string>>();
    foreach (var line in lines)
    {
        var tokens = line.Split(':', 2);
        var id = tokens[0];
        var text = tokens[1];
        if (dict.ContainsKey(id))
            dict[id].Add(text);
        else
            dict[id] = new List<string> {text};
    }

    return dict;
}

{
  "1": [
    "a",
    "b"
  ],
  "2": [
    "c"
  ],
  "3": [
    "d",
    "x"
  ],
  "biba": [
    "boba:buba"
  ]
}

StringBuilder sb = new StringBuilder();
sb.Length--; //удаляем последний символ