System.InvalidOperationException: 'There are multiple navigations in entity type 'Post' which are pointing to same set of properties 'Account' using a [ForeignKey] attribute: 'PostParticipantsIds', 'UsersAttendingIds'.'

System.InvalidOperationException: 'There are multiple navigations in entity type 'Post' which are pointing to same set of properties 'Account' using a [ForeignKey] attribute: 'PostParticipantsIds', 'UsersAttendingIds'.'

using Microsoft.AspNetCore.Identity;

namespace CommunityWebsite_Lexicon_Project.Models
{
    public class Account : IdentityUser
    {
        public int Id { get; set; }
        public string? UserName { get; set; }
        public string? Email { get; set; }
        private string Password { get; set; }
        public async Task SetPassword(string password)
        {
            Password = new PasswordHasher<Account>().HashPassword(this, password);
        }

        public async Task<bool> VerifyPassword(string password)
        {
            var result = new PasswordHasher<Account>().VerifyHashedPassword(this, Password, password);
            return result == PasswordVerificationResult.Success;
        }
    }
}

using Microsoft.AspNetCore.Identity;

namespace CommunityWebsite_Lexicon_Project.Models
{
    public class Account : IdentityUser
    {
        public int Id { get; set; }
        public string? UserName { get; set; }
        public string? Email { get; set; }
        private string Password { get; set; }
        public async Task SetPassword(string password)
        {
            Password = new PasswordHasher<Account>().HashPassword(this, password);
        }

        public async Task<bool> VerifyPassword(string password)
        {
            var result = new PasswordHasher<Account>().VerifyHashedPassword(this, Password, password);
            return result == PasswordVerificationResult.Success;
        }
    }
}

using System.ComponentModel.DataAnnotations;
using System.Xml.Linq;

namespace CommunityWebsite_Lexicon_Project.Models
{
    public class LoginModel
    {
        public string? Username { get; set; }
        public string? Email { get; set; }
        public string? Password { get; set; }
        public string? PasswordConfirm { get; set; }
    }
}

using System.ComponentModel.DataAnnotations;
using System.Xml.Linq;

namespace CommunityWebsite_Lexicon_Project.Models
{
    public class LoginModel
    {
        public string? Username { get; set; }
        public string? Email { get; set; }
        public string? Password { get; set; }
        public string? PasswordConfirm { get; set; }
    }
}

public async Task<IActionResult> Register(LoginModel submittedAccountModel)
        {
            if (ModelState.IsValid)
            {
                Account createdUser = new Account() // Create account object.
                {
                    UserName = submittedAccountModel.Username,
                    Email = submittedAccountModel.Email,
                };

                Regex regex = new Regex(@"^[a-zA-Z0-9]+$"); // Only letters and digits allowed.
                if (!regex.IsMatch(createdUser.UserName))
                {
                    ModelState.AddModelError("NickName", "Only letters and digits are allowed in the NickName field.");
                }

                try // Setting the password.
                {
                    await createdUser.SetPassword(submittedAccountModel.Password);
                } catch (Exception)
                {
                    throw new Exception("Error.");
                } finally // Verify the password.
                {
                    await createdUser.VerifyPassword(submittedAccountModel.PasswordConfirm);
                }

                IdentityResult result = await _userManager.CreateAsync( // Create the Account-Identity object.
                    createdUser, createdUser.PasswordHash
                    );

                if (result.Succeeded)
                {
                    await _accountRepository.AddAsync(createdUser); // Save the Account-Identity object.
                    return RedirectToAction("Login");
                    //return Ok();
                } else
                {
                    return View(submittedAccountModel);
                }
            } else
            {
                return BadRequest();
            }
        }

public async Task<IActionResult> Register(LoginModel submittedAccountModel)
        {
            if (ModelState.IsValid)
            {
                Account createdUser = new Account() // Create account object.
                {
                    UserName = submittedAccountModel.Username,
                    Email = submittedAccountModel.Email,
                };

                Regex regex = new Regex(@"^[a-zA-Z0-9]+$"); // Only letters and digits allowed.
                if (!regex.IsMatch(createdUser.UserName))
                {
                    ModelState.AddModelError("NickName", "Only letters and digits are allowed in the NickName field.");
                }

                try // Setting the password.
                {
                    await createdUser.SetPassword(submittedAccountModel.Password);
                } catch (Exception)
                {
                    throw new Exception("Error.");
                } finally // Verify the password.
                {
                    await createdUser.VerifyPassword(submittedAccountModel.PasswordConfirm);
                }

                IdentityResult result = await _userManager.CreateAsync( // Create the Account-Identity object.
                    createdUser, createdUser.PasswordHash
                    );

                if (result.Succeeded)
                {
                    await _accountRepository.AddAsync(createdUser); // Save the Account-Identity object.
                    return RedirectToAction("Login");
                    //return Ok();
                } else
                {
                    return View(submittedAccountModel);
                }
            } else
            {
                return BadRequest();
            }
        }