I do not suggest 2x dosing this.
Follow the label(it's there for good reason).
The topic here is more than just dosing excel, the owner has algae, diatoms and then greens coming in.
The root issue here is CO2 more than likely.
We can easily add ferts and do water changes to make sure that's in a good range, light is assumed to be stable, on for 10 hours 2-3 w/gal etc(?).
So then the rest is a good cleaning, filter, tank, pruning the algae that's there etc.
Then you tweak the CO2(while you are able to see the tank, do not add more then wander off for the rest of the day).
Look at plant growth. pearling etc, ignore the algae for a bit.
Since most folks are in a hurry to kill algae, adding more Excel, like using one huge hammer is not wise. Better to add the 5mls per 40 liter suggested dosing.
Now you can use several smaller hammers together to do the job and about as quick and never harm any fish:
Do the 3 day black out, cover tank with garbage bags etc, turn off CO2.
Do daily 50-60% water changes, add Excel back at 5-6 mls per 40 liters.
At the end, add ferts back, turn CO2 back on and tweak the CO2.
This will kill most algae species.
Unless you plan on using Excel from here on, or unless you plan on reducing the light intensity, you need to really focus on CO2 gas dosing.
Some use observational skills + test kits.
I use observational skills mostly.
I add just enough for excellent pearling and plant growth.
Any lull in plant growth, I check and add a tad more CO2 to see.
A lull in plant growth is a warning sign and proceeds an algae issues which tends to be a symptom of a more intense serious CO2, or nutrient issues.
So watch the plants carefully, and tweak them, then you stay ahead of algae.