Identity ChangePasswordAsync returns DuplicateUserName

Question

Thank you for taking the time and reading this.

I've searched everywhere but didn't find the answer to this problem :(

public async Task<IEnumerable<IdentityError>> ChangePassword(ChangePasswordCommand model, string userId)
        {
            try
            {
                var user = await _userManager.FindByIdAsync(userId);

               if (user == null)
                    return null;

               var checkPasswordResult = await _userManager.CheckPasswordAsync(user, model.Password);
                
                var result = await _userManager.ChangePasswordAsync(user, model.Password, model.NewPassword);

               if (!result.Succeeded)
                    throw new ValidationException();

               return new List<IdentityError>();
            }
            catch (Exception)
            {
                throw;
            }
        }

This thing returns DuplicateUserName.

When I add this line before ChangePasswordAsync():

_userManager.UserValidators.Clear()

It works.

How is the error connected to the password change operation?