I imported the games from GOG by doing bulk import using 2 lists, one for Windows games and one for Linux games. I figured this would make it so that Linux would be added to the games that were already imported in the previous set but instead it duplicated the entries.

Did I miss some toggle/setting when doing the second import to make it so that Linux would be added to the previously imported game instead of creating a separate entry just for the Linux version?

Is it not possible to set multiple platforms on one game entry?

No, the way the system is set up is that each game entry is tied to a single platform. Makes it possible to handle having different sets of info (purchase price, acquisition source, or any physical properties for physical games, etc...) if you own multiple copies of the same game.

Then is there a way to group/filter out these duplicate entries (yes, I say duplicate because for all intents and purposes that is what those are due to the game, edition/version, format and everything else being literally the same except for the Platform) ?

Having Windows (besides being grouped with PC) as separate platform from Mac and Linux doesn't make sense because Mac is still PC (Personal Computer) based, it's not a console or using special hardware configurations different from PCs running Windows (intel/amd CPUs, video card, motherboard, etc.) and Linux is also not a separate platform, rather just a different operating system (software) running on the same platform (hardware) as Windows.

From the My Collection page, there isn't a way to filter out duplicates. There is a Duplicate Items report under the Reports menu, but I don't think that's what you're looking for.

One potential solution for you would be to utilize the "hidden" setting. When you edit an item in your collection, you should see a checkbox labeled "Hidden". By default, items marked with this option are NOT displayed when viewing your collection.

Another option you could look at would be to have a single entry for a game in your collection and then enter a tag for additional operating systems it might support. Tags can be entered in bulk and you can filter on them, so they might help in your scenario.

The hidden thing is what I was doing with various games that I got via PS+ where they'd give cross-plat ps3, ps4, and/or vita version of the same game. I'd keep the most relevant one as the unhidden entry, and hide the other two. More work as you're creating 2-3 entries, but it all depends on how much detail you want in your list.

Regarding having Windows / Mac / Linux as separate platforms:

A lot of the site features that exist now didn't always exist. Like the tagging system (which could be used to tag a generic "PC" platform game with the specific OS it runs on) in lieu of having Windows, Mac and Linux as their own independent faux platforms. Before tagging, notes, etc... there wasn't a really good way for users to organize different OS compatible games in their collections, so Mac and Linux platforms were added. No, it's not the most accurate thing in the world to call them "platforms", but it was a quick and easy way to add support for something that some users had asked for. As to why Windows is still lumped in with PC in the platform list, I can only presume it's because most people associate PC gaming with Windows gaming, and vice versa. Market share and all that.

At least, I'm pretty sure that's how things came about. My memory is spotty at the best of times and moho_00's the one that actually does everything. I'm just a long-time user. :)

So, would you be open to changing the way things are done or are you set in continuing to implement and suggest workarounds in order to keep supporting legacy implementations/design decisions?

I would see the following changes as being massive improvements for not only the user experience but also for keeping a fit database without unnecessary duplication/redundancy:
- Have each game appear only once in a library by defining it as a base entity with fields/properties that are common to all editions/versions.
- All editions/extensions/variations of a game would be linked to the game but be treated as separate entities only at the database level, thus allowing challenges and version specific information to be tracked per edition/version while keeping the library free from duplicates and having to use workarounds.
- Visually one could make it so that logos/names of each platform the game is owned on would be shown on the cover art when one hovers over it. On the list view, there would be a column displaying all platforms and one for displaying all editions the user has added of that game.

If you are interested in my suggestions I would be glad to offer support to seeing it through and even provide help with the actual implementation if that is an option.
I don't see a github/gitlab link mentioned anywhere so I don't know if you're open to actual code contributions or if you're determined to work solo on the coding side and just have moderators and contributors use an admin interface for some managerial duties.

In any case I'm interested in seeing where this project is heading and if you're open to reworking core parts of the project.

Having operating systems loaded as separate platforms is in-line with other collection / gaming sites and I think can be important for physical collection purposes. Mac and Linux were added very early on, though I don't recall if it was before or after the site became available to the public. While I agree that Mac and Linux are also technically running on a "PC", Windows is often (for better or worse) synonymous with that term. One could argue that we should rename "PC / Windows" to simply "Windows" (it was originally just called "PC").

I'm always open to improving the site, but compiling multiple instances of the same item into a single entry in your collection is not currently on my radar. This has been suggested in the past and it's something I've considered and would love to add at some point. There are some architectural challenges that would need to be handled (on the UI side), as well as things like when you add a compilation that contains a game you had manually added (these have special restrictions). Or when you add the same game for the same platform (especially prevalent with different digital providers on PC.) Certainly all things that could be worked through, but ultimately, time is limited and my focus is on other features at the moment (primarily filling out and enhancing the Browse by Platform page.)