Yes and no.So investing in the same item over and over again will yield increasing returns? Like I already have 1 of each, that's why I am asking.
The items and equipment you get from the Contracts work that way, so if you want more of the same piece of equipment or you're going through a boatload of potions, that's a good way to get more of them as you progress.
The Silcrow return doesn't change though. So it doesn't matter if you have 1, 5, or 100 of a certain Contract, it'll only count towards your return the once.
The specific numbers for what you get from each Contract is listed in the Contracts tab of Estate Info in the menu.