Working out twice a day is quite typical when reaching advanced and elite levels. When people ask this question, I constantly make reference to olympic weightlifters, who lift heavy hours per day, often twice or three times per day.
Practically every elite (strength-related) athlete lifts or utilizes their muscles to a similar extent multiple times per day. Take professional football players for example: weightlifting and practice almost every day. There are some exceptions to this rule, but those are folks who are lifting at inconceivably large tonnages (if you're squatting 800+, this is you).
I love this quote by John Broz, coach of 2011 and 2012 US national weightlifting champion Pat Mendes:
If you can't go into the gym and squat heavy twice a day, every day,
you aren't overtrained, you're undertrained.
Love him or hate him, I think Greg Glassman hit the nail on the head with this quote as well:
Why is it that those most inclined to worry and ask about
“overtraining” are about as likely to set a new record in the Olympic
Decathlon as they are to ever overtrain?
The main concern is injury. Injuring (straining) muscle tissue is a concern, but more important are the other soft connective tissues which can take months to heal. Cartilage and discs of the spine are also easily damageable when overdoing it. So it's important to gradually increase your volume.
To answer your question, I would imagine the additional volume was beneficial, at least in acute bouts such a single day. It generally takes a long time to start seeing positive or negative effects of modifying training volume and frequency. If you want a legitimate answer, you need to be scientific about it and measure. You know, stick to twice-per-day on your arm workout for six weeks and see if your strength improves. Get a measuring tape and measure your upper arms. That sort of thing.
Frequency is just another variable in the progressive overload equation, which, by the way is a pretty complex equation once you start being scientific about it. Bret Contreras lists these 12 methods of progressive overload in an article, a couple of which are relevant to your question:
- Lifting the same load for increased distance (range of motion)
- Lifting the same load and volume with better form, more control, and
less effort (efficiency)
- Lifting the same load for more reps (volume)
- Lifting heavier loads (intensity of load)
- Lifting the same load and
volume with less rest time in between sets (density)
- Lifting a load
with more speed and acceleration (intensity of effort)
- Doing more
work in the same amount of time (density)
- Doing the same work in less
amount of time (density)
- Doing more sets with the same load and reps
- Lifting the same load and volume more often throughout the
- Doing the same work while losing body mass
(increased relative volume)
- Lifting the same load and volume and then
extending the set past technical failure with forced reps, negatives,
drop sets, static holds, rest pause, partial reps, or post-exhaustion
(intensity of effort)
I'm sure, especially at 23, it was almost a non-issue for your body. Just get enough rest and eat. Here's some further reading if you're interested: