I think it has to do with the amount of time the dechlor will remain effective in a tank until it is either used up neutralizing or because it loses its ability to dechlorinate. But, it is rare I can see the need to double dose intentionally.
Persnally, I would think that it matters how large a change one is doing. I would not dose the entire tank volume is I was only changeing10 or 20%. Different brands do claim differing times for the persistence of their product in the water and its being able to do its job.
While there is no need to over dose most things we might add to a tank, when it comes to chlorine it is better to err on the side of using too much rather than too little dechlor. But double seem too much. However, I also believe that the companies who make the products we use to kill our fish. Give all the years and useage of dechlors, I have to assume that overdosing it within reason should not harm most fish. If this did we hobbyists would have figured that out long ago.
Consider that most of us do not know he exact volume of water in any of our tanks. Anything in the tanks not water means that much less water than can fit. So substrate, rocks and wood all displace water and reduce the tank's capacity for holding water. Then, most of us do not fill that tank to the point it would overflow should much more water be added, On my rimmed tanks I want the water line to be above the edge of the frame so it is not visible. That means there is space between the water and the lid.
But when we remove the water from and established tank with substrate and other decor, most of us have no real idea what % of the water we have removed.
When I do fish events and use 50/50 my RO/DI water and half the hotel's water, I estimate the dechlor when I add it. I tend to be sure it is enough which likely means it is more than needed. I do not lose fish at events.