So I have been doing an intense workout in the afternoon, which involves the triceps and the shoulders. I have done this workout to failure for about an hour, until I couldn't do any more reps. Approximately 5-6 hours later, I tried doing the same workout and I notice that I don't struggle and do it quite easily. Does this mean that I should continue going? Is it effective to do the same workout to train the same muscles a couple of hours later, if it goes well? Are there any downsides to this? I suppose this gives you the maximum hypertrophy right?
There are a few considerations and are based on what you just described (and assuming you have no other medical conditions or anything affecting recovery be it hindrance etc.). These are just some comments based on your question and in no way constitute a consultation.
The fatigue (from the to failure) could be from a multiple of reasons: e.g. neuromuscularly (brain-muscle) or there is too much lactate (not a bad thing - just not the best for this) or simply lack of energy. And if you can "recover" that fast simply means that the fatigue is more of the former and the load/intensity might not be as heavy as you think to trigger all the physiological process associated with hypertrophy.
You should also check if you are able to produce the same level of force as before, which you suggested you are, because that is what fatigue is. You may not feel the DOMS immediately but still....
What you alluding to is "Time under tension". It is one of the most important concepts in hypertrophy. While total volume (time) is important, the quality (intensity/load) is just as important (especially since you want it to trigger all the relevant physiological processes). Think about it, if volume or time under tension is the only important consideration, we will have huge power walkers/marathoners with huge legs, because their training volume and the appropriate time under tension is huge.
My suggestion, see a qualified Coach, get proper training load/intensity calculations and always feedback to the coach so that you can get the best safe and optimal training load. instead of just "burning calories".
For a beginner and intermediate, I don't think so. The advanced body builder has a special plan and knows perfectly what he is doing, but for beginner and intermediate, the most effective way to gain weight is to do an intense heavy weight lifting (with low reps) and then allow for full recovery (muscles grow in sleep and when you are resting). So it is better to let these muscles rest and also get a day off. In my youth, I was training several times per day every day and it was too much (I could get the same results with much less training). If you feel like you could still do the same training again, just increase the intensity of your training by increasing weights or adding more sets (up to 5 I would say).