AppHost.cs

public override void Configure(Container container)
{
    container.RegisterAs<OneSessionAuthEvents, IAuthEvents>();
}

OneSessionAuthEvents.cs

using System.Collections.Generic;
using System.Linq;
using ServiceStack;
using ServiceStack.Auth;
using ServiceStack.Caching;
using ServiceStack.Web;

namespace Orca.ServiceInterface.Authentication
{
    public class OneSessionAuthEvents : AuthEvents
    {
        public override void OnAuthenticated(IRequest httpReq,
                                             IAuthSession session,
                                             IServiceBase authService,
                                             IAuthTokens tokens,
                                             Dictionary<string, string> authInfo)
        {
            using (var cache = authService.TryResolve<ICacheClient>())
            {
                var sessionPattern = IdUtils.CreateUrn<IAuthSession>("");
                var sessionKeys = cache.GetKeysStartingWith(sessionPattern).ToList();
                var allSessions = cache.GetAll<IAuthSession>(sessionKeys);
                var existingSessions = allSessions.Where(s => s.Value.UserName == session.UserName);
                cache.RemoveAll(existingSessions.Select(s => s.Key));
            }
        }
    }
}