I have yet to find a grocery store that will double a coupon, with the exception of one in my region. The one store that did offer to double a coupon limited to one, and had a limit to the amount. I have to wonder why a person would want 5 years worth of diapers, if a person doesn't have a child.
I also doubt anyone of these people count in the cost of storage for the volume of their hoard either, or the cost of travel to and from the stores. The products are taking up space in their homes, that could often used in a more productive manner. I have also found that most products tend to have a pull date on them. Even bath soap will loose it's scent over time. Items like deodorant will loose it's effectiveness overtime also. It is difficult and expensive to provide an environment that allows a private party to store most things for extended periods of time.
That said, I do personally tend to buy for a year at a time. I have overtime, found that some items are much cheaper or on sale at specific times of the year, Right now the grocery stores have baking items at the lowest point of the year. So I stock up on things like flour, sugar, butter, and like items. I also watch for clearance sales after the sale or holiday. But I try to never buy more than can be used during the year.
But the best way to really save money is stay out of the store in the first place. If a person can get a great deal on something like deodorant, or bath soap, or toothpaste, they need to know that the homeless shelters are always in need.
PS, I do buy my bath soap from a maker on the Hawaiian Island. I usually buy enough that one time so that I maximize the shipping. I also watch for free shipping offers.