1. It would hurt the sales of GoG
It might, but its better for the consumer and in turn the 'good will' will be better for GoG in the long run.
Purchasing a duplicate copy of a game is generally 'wasted money'. There is very little point to owning a game twice. (There will be reasons why someone might want duplicate copies but I would bet its not the norm).
GoG might miss out on the sale of that particular game but it also means the consumer still has that money available for something else which could potentially mean that money is spent on another game (non duplicate) at GoG.
2. Technically speaking, the store page shows you what you already own of GoG products. That is its scope and it is doing it properly.
I understand that, and I'm guessing its just pulling the data from the website and isn't integrated into Galaxy very much at all, however the possibility is still there to have the feature added.
3. The GoG Store is not selling the same version of games you have anywhere else because they are always DRM-free version
I would suspect that when it comes to duplicate copies of a game DRM isn't much of a deciding factor. The consumer already owns the game on another platform.
It might have been by choice because they don't care about the DRM, it might be because it wasn't available on GoG at the time, or maybe its an old game that was purchased before GoG was around.
Regardless, the consumer should be able to make an
informed decision as to whether they want to repurchase the game from GoG.
Programs like Galaxy 2.0 (where your games are aggregated into a single launcher) aren't new.
You have software like Launchbox and Playnite... Hell, AMD even tried it ages ago with their AMD Gaming Evolved software (re-branded Raptr).
The reason programs like this came into existence was because of the fragmentation of the gaming market.
You have so many launchers and market places now its hard to keep track of what you own and what you don't.
So on one hand, by aggregating your games into a single launcher GaG Galaxy 2.0 is trying to combat the problem, and on the other hand, they are contributing to the fragmentation issue by not showing games owned on another platform... Which to me seems counter intuitive in a piece of software designed to aggregate your games.
Its not something I would expect when browsing the store through the website, but there's no excuse for not having it within the Galaxy 2.0 software (other that trying to take advantage of the consumer purchasing duplicates).