GetClaimsAsync(user) throws Sequence contains more than one matching element

I have a .NET Core 2.0 Web API. I am using Jwt authentication on it.

Whenever the application reaches this line, an exception is thrown:

var userClaims = await _userManager.GetClaimsAsync(user);

The exception is:

System.InvalidOperationException: Sequence contains more than one matching element

What is weird is that this wasn't happening before, it started happening when I upgraded to .NET Core 2.0 from 1.1.

The Seed method for the database is below:

public async Task Seed()
{
  var adminUserJ = await _userManager.FindByNameAsync("Ciwan");
  var regularUser = await _userManager.FindByNameAsync("John");

  if (adminUserJ == null)
  {
    if (!await _roleManager.RoleExistsAsync("Admin"))
    {
      var role = new IdentityRole("Admin");
      role.Claims.Add(new IdentityRoleClaim<string> { ClaimType = "IsAdmin", ClaimValue = "True" });
      await _roleManager.CreateAsync(role);
    }

    adminUserJ = new BbUser
    {
      UserName = "Ciwan",
      Email = "[email protected]"
    };

    var userResult = await _userManager.CreateAsync(adminUserJ, "Welcome123");
    var roleResult = await _userManager.AddToRoleAsync(adminUserJ, "Admin");
    var claimResult = await _userManager.AddClaimAsync(adminUserJ, new Claim("Points", "10"));

    if (!userResult.Succeeded || !roleResult.Succeeded || !claimResult.Succeeded)
    {
      throw new InvalidOperationException("Failed to build user and roles");
    }
  }

  if (regularUser == null)
  {
    if (!await _roleManager.RoleExistsAsync("Regular"))
    {
      var role = new IdentityRole("Regular");
      role.Claims.Add(new IdentityRoleClaim<string> { ClaimType = "IsRegular", ClaimValue = "True" });
      await _roleManager.CreateAsync(role);
    }

    regularUser = new BbUser
    {
      UserName = "John",
      Email = "[email protected]"
    };

    var userResult = await _userManager.CreateAsync(regularUser, "BigWow321");
    var roleResult = await _userManager.AddToRoleAsync(regularUser, "Regular");
    var claimResult = await _userManager.AddClaimAsync(regularUser, new Claim("Points", "10"));

    if (!userResult.Succeeded || !roleResult.Succeeded || !claimResult.Succeeded)
    {
      throw new InvalidOperationException("Failed to build user and roles");
    }
  }

  _context.AddRange(GetListOfArtists(adminUserJ.Id, regularUser.Id));
  await _context.SaveChangesAsync();
}

I can't see anything wrong. I tried looking at the AspNetUserClaims table in the database, but all seems OK. I have 2 claims in there, one for each user.

This error happens when I attempt to log in a user, so the request arrives here:

[HttpPost("auth/token")]
public async Task<IActionResult> CreateToken([FromBody] CredentialsDto credentials)
{
  try
  {
    var user = await _userManager.FindByNameAsync(credentials.Username);
    if (user != null)
    {
      if (IsUserPasswordValid(credentials, user))
      {
        var claims = await GetClaimsAsync(user);
        var token = CreateNewJwtToken(claims);

        return Ok(new
        {
          token = new JwtSecurityTokenHandler().WriteToken(token),
          expiration = token.ValidTo
        });
      }
    }
  }
  catch (Exception exception)
  {
    Console.WriteLine(exception);
    throw;
  }
  return BadRequest("Failed to generate token!");
}

private async Task<IEnumerable<Claim>> GetClaimsAsync(BbUser user)
{
  var userClaims = await _userManager.GetClaimsAsync(user);
  return new[] {
          new Claim(JwtRegisteredClaimNames.Sub, user.UserName),
          new Claim(JwtRegisteredClaimNames.Jti, Guid.NewGuid().ToString())
  }.Union(userClaims);
}