While double dosing in general may not be a problem with ivermectin, it's not something you want to just do because there are situation where even at normal doses ivermectin can cause neurological toxicity. Certain plants if eaten can lead to this, any injury to the brain/spinal cord (including trauma, wobblers, EPM, any ecephalytic disease, etc), possibly even gastric ulceration.
So, while it's been tested safe in healthy horses it doesn't mean that you should just give it that way all the time.Because when you get one of those surprises where it does cause problems like your horse having picked a bit of bull nettle, then you've created an even bigger problem by double dosing.
There really is no reason to double dose ivermectin because it doesn't make it kill any more parasites. But if it's done accidentally, the risk of it causing problems is pretty small.
Licensed Veterinary Technician