I did not take into account what your race distance or age might be. I was thinking only of myself...a working adult training for the half/full marathon distance. If you are in high school, and especially if you are training for shorter distances maybe there is room for doubles when your average daily distance is < 10 miles. Anyway, this is my response to "dumbest advice" guy...
http://kemibe.com/distancecoach/labreports/twoadays.shtml
"Runners often start running twice per day before weekly mileage warrants it. Doing “doubles†sounds like serious training, so it must be better preparation. In specific situations, this is true. Most runners, however, should resist the urge to switch from single runs to doubles as training mileage increases. Let’s take a look at when double workouts are and are not beneficial, and how to add doubles to your training program.
A basic principle to follow is to not do double workouts until you have maximized the amount you can handle in single workouts. Staying with longer single runs builds endurance, while shorter doubles allow you to train at a faster pace. Double runs can also be beneficial in speeding recovery. Two short runs will help you recover more quickly than one longer run.
The weekly mileage at which you should add double workouts to your training schedule depends on the distance you race (see table below). The longer the race, the more your training should focus on endurance-based adaptations such as depleting your glycogen reserves to provide a stimulus for your body to store more glycogen, and training your muscles to utilize more fat at a given speed. Logically, you will provide a greater stimulus for these adaptations through a single 12 mile run than by doing a 7 mile and a 5 mile run at the same pace.
As shown in the table, if you are preparing for a marathon and are running less than 75 miles per week, then you should not be running doubles. After you schedule your long run and a mid-week medium long run, there is really no reason to double to get in the remaining miles. Once your marathon training calls for more than 75 miles per week, however, there is a definite role for double workouts in your program.
The shorter the race that you are preparing for, the lower the mileage at which you should add double workouts. If you are preparing to race 5 km, for example, your interval workout is the most important training session of the week, and you will need to keep your legs fresh. You will also want to maintain a faster pace during some of your normal training runs, which is accomplished more easily during two short runs than one longer run.
When to add double workouts
Race distance prepared for Weekly mileage exceeds
5 km or less 55
8 km to 12 km 60
15 km to 25 km 70
30 km to marathon 75"
If you don't consider Pete Pfitzinger a reliable source......
https://runnersconnect.net/coach-corner/double-runs/
"Generally, doubles will benefit more experienced runners who have maximized aerobic development but still need to boost mileage without having their easy days be over 80-90 minutes or who are looking to recover more efficiently in preparation for harder workouts.
Here’s a brief breakdown of what type of mileage numbers you should be at before adding doubles to your training regimen:
Primary Event.....Average mileage
1500m – 5k........55 – 60 mpw
8k – 10k.............65 – 70 mpw
Half Marathon....70-75 mpw
Marathon...........75-80 mpw"
I stand by my response, though it seems I could have said it more softly.