ClaimsEndpoint.cs

using Microsoft.AspNetCore.Mvc;
using System.Collections.Generic;
using System.Linq;

[ApiController]
public class ClaimsEndpoint : ControllerBase
{
	[HttpGet("/uwu")]
	public ActionResult Handle()
	{
		return Ok(new
		{
			Identities = User.Identities.Select((i) => new
			{
				i.AuthenticationType,
				Claims = i.Claims
					.GroupBy((c) => c.Type)
					.Select((g) => KeyValuePair.Create<string, object>(
						g.Key,
						g.Count() is 1 ? g.First().Value : g.Select((c) => c.Value)
					))
					.ToDictionary()
			}),
		});
	}
}