.NET 6

Program.cs

var dataLoader = new DataLoader();

Console.WriteLine("Executing...");

var t1 = dataLoader.LoadAsync("1");
var t2 = dataLoader.LoadAsync("1");
var t3 = dataLoader.LoadAsync("9999");

Console.WriteLine("Done Loading...");

Console.WriteLine(t1 == t2);
Console.WriteLine(t1 == t3);

Console.WriteLine(await t1);
Console.WriteLine(await t2);
Console.WriteLine(await t3);

public class DataLoader
{
    private readonly SemaphoreSlim semaphoreSlim = new SemaphoreSlim(1, 1);

    private Dictionary<string, Task<string>> Data { get; set; }

    public DataLoader()
    {
        this.Data = new Dictionary<string, Task<string>>();
    }

    public Task<string> LoadAsync(string id)
    {
        this.semaphoreSlim.Wait();

        try
        {
            if (this.Data.TryGetValue(id, out var dataTask))
            {
                return dataTask;
            }

            dataTask = GetString(id);

            this.Data.Add(id, dataTask);

            return dataTask;
        }
        finally
        {
            this.semaphoreSlim.Release();
        }
    }

    private async Task<string> GetString(string str)
    {
        await Task.Delay(5000);

        return str;
    }
}