OWIN ASP.NET - Avoid multiple logins for the same account without Identity in Web Api

I want to know how can I block users to have multiple Refresh Tokens working at the same time. Let me explain:

• Some user ask for a new Access Token using Credentials.
• Credentials are okey, so the Authentication servers returns an Access Token and a Refresh Token.
• When Access Token expires, user have to use the new Refresh Token instead of Credentials to get a new Access Token.

The problem is that if another user log in using the same credentials, another Refresh Token will be generated for the same identity. So, what i want to do is: if somebody log in again using some credentials that have an active refresh token, instead of generating a new one, replace the existing one, or delete it and insert the new one. So the previous user will be disconnected when Access Token expires since Refresh Token wont exist anymore.

Also, how can i implement some service to destroy a Refresh Token in the Authentication Server? So the user can call it to disconnect his account, not just deleting the cookie and wait till it expires.

Here is my code:

Startup.cs:

public partial class Startup
{
    public void Configuration(IAppBuilder app)
    {
        HttpConfiguration config = new HttpConfiguration();

        config.MapHttpAttributeRoutes();
        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}"
        );

        app.UseOAuthAuthorizationServer(new OAuthAuthorizationServerOptions()
        {
            AllowInsecureHttp = true,

            TokenEndpointPath = new PathString("/auth"),
            AccessTokenExpireTimeSpan = TimeSpan.FromMinutes(1),

            Provider = new OAuthProvider(),
            RefreshTokenProvider = new RefreshTokenProvider()
        });
        app.UseOAuthBearerAuthentication(new OAuthBearerAuthenticationOptions());
        app.UseWebApi(config);
    }
}

OAuthProvider.cs:

public class OAuthProvider : OAuthAuthorizationServerProvider
{
    public override Task ValidateClientAuthentication(OAuthValidateClientAuthenticationContext context)
    {
        context.Validated();
        return Task.FromResult<object>(null);
    }

    public override Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
    {
        try
        {
            var account = AccountRepository.Instance.GetByUsername(context.UserName);
            if (account != null && Global.VerifyHash(context.Password, account.Password))
            {
                var claimsIdentity = new ClaimsIdentity(context.Options.AuthenticationType);
                claimsIdentity.AddClaim(new Claim(ClaimTypes.Name, account.Username));
                claimsIdentity.AddClaim(new Claim("DriverId", account.DriverId.ToString()));

                var newTicket = new AuthenticationTicket(claimsIdentity, null);
                context.Validated(newTicket);
            }
        }
        catch { }

        return Task.FromResult<object>(null);
    }

    public override Task GrantRefreshToken(OAuthGrantRefreshTokenContext context)
    {
        context.Validated();
        return Task.FromResult<object>(null);
    }
}

RefreshTokenProvider.cs:

public class RefreshTokenProvider : AuthenticationTokenProvider
{
    public override Task CreateAsync(AuthenticationTokenCreateContext context)
    {
        var refreshToken = new TokenModel()
        {
            Subject = context.Ticket.Identity.Name,
            Token = GenerateToken(),
            IssuedUtc = DateTime.UtcNow,
            ExpiresUtc = DateTime.UtcNow.AddMinutes(5)
        };

        context.Ticket.Properties.IssuedUtc = refreshToken.IssuedUtc;
        context.Ticket.Properties.ExpiresUtc = refreshToken.ExpiresUtc;

        refreshToken.Ticket = context.SerializeTicket();

        try
        {
            TokenRepository.Instance.Insert(refreshToken);
            context.SetToken(refreshToken.Token);
        }
        catch { }

        return Task.FromResult<object>(null);
    }

    public override Task ReceiveAsync(AuthenticationTokenReceiveContext context)
    {
        try
        {
            var refreshToken = TokenRepository.Instance.Get(context.Token);
            if (refreshToken != null)
            {
                if (TokenRepository.Instance.Delete(refreshToken))
                {
                    context.DeserializeTicket(refreshToken.Ticket);
                }
            }
        }
        catch { }

        return Task.FromResult<object>(null);
    }

    private string GenerateToken()
    {
        HashAlgorithm hashAlgorithm = new SHA256CryptoServiceProvider();

        byte[] byteValue = Encoding.UTF8.GetBytes(Guid.NewGuid().ToString("N"));
        byte[] byteHash = hashAlgorithm.ComputeHash(byteValue);

        return Convert.ToBase64String(byteHash);
    }
}

One more question: how can i throw an Internal Server Error in the catch? Because it's actually returning invalid_grant, but the catch means a Database error, not invalid Credentials or Token.

Thanks for the help, and sorry for the bad english. I hope you understand!