I'm still young enough to remember things.

I don't see any problem of keeping track.
For starters, you use your brain :P I have a lot of games over 13 platforms and I know them all by heart. I do have some titles duplicated or triplicated or quadruplicated, but all intentionally.
Secondly, since you mention GOG and Steam, how about just checking the other one when you plan to make a purchase on one to see if you already have it. Also good to check pluses and minuses for each title and decide where it is better to buy.
Next up, use Excel or some other spreadsheet program. Write all titles alphabetically. One column for GOG, one for Steam. After each purchase, add to the spreadsheet.

Easy. I don't have Steam. :P

I use a password manager for that and new accounts are never an issue.

I'm pretty much old school when it comes to this, trying to learn all my passwords by heart. Which leads to me having to have them re-sent to me pretty often xD

I'll second Collectorz. I have slightly over 700 games across steam, gog and retail. If your not worried about future updates you can just pay the once to get access to it. It has some nice functionality with being able to search for games and have alot of the information downloaded and added automatically including a cover picture (box art or otherwise). You're also able to edit your entries and they have fairly recently added a bunch of user text fields.

It also allow some customer filters, which i've now setup to show my gog and steam librarys seperatly at the click of a button. There is a 50 game limited trial if it sounds like something you might be intrested in.

I had the same problem, plus with all of the console game collecting I do (specifically pre-2000 nostalgia stuff). I tried a number of site and there are some good ones out there (like Backloggery), but I couldn't find one that met all of my criteria. Since I'm a web developer and I enjoy programming so much, I built my own, which some of you may have seen before :D You can check it out here: http://www.completionator.com/

Here's a link to my collection, in case you're curious what it looks like: http://www.completionator.com/Collection/Index/1

Steam was a big problem for me since I have so many games on it, so I ended up building an import, which has worked out very well so far. I'm working on ideas for getting something similar for GOG, but the last time I checked, they don't provide an API that I can access, so I'll have to get creative.

Anyways, feel free to check it out and let me know what you think! I'm on a stay-cation right now working on my backlog, but I'll be picking up development on it again on 1/9/15.

How Long To Beat is your friend... as well as you being able to keep a list of games you own, are playing, have completed or retired, you can also see how long your games might take to complete, as well as submit your own playtimes, write reviews and generally be involved in a nice respectful and generous gaming community - possibly the best I've been involved with.

http://www.hl2b.com

I use a simple spreadsheet that I update whenever I buy a new game. It has what store the game is on, what platforms the game runs on, have I managed to make it run on a Mac if it doesn't normally, how far into the game am I, and what do I generally think of it so far. Works like a charm.

If I knew how much time it would take to complete my backlog, I think I'd be very depressed.

Why yes, you can have Excel sort automatically!

The first way is to have Excel sort as soon as you enter your data:

http://excel.tips.net/T009005_Automatically_Sorting_as_You_Enter_Information.html

However, I don't recommend this as once you hit enter it's sorted and there's no going back, (meaning you can't use Ctrl-Z to undo it.)

The other option, which I've used in some of my sheets is to have the list automatically sorted when the worksheet is opened:

http://www.mrexcel.com/forum/excel-questions/254433-auto-sort-opening-workbook.html

If the code doesn't work, you can always record the macro, copy the code when finished, and paste it into the specific area used for the Workbook_Open event.

As for tabs, I don't think this is the best way to handle your data since an important objective is to search quickly for if you have previously purchased a game. You would have to set the Excel search to search all worksheets and this causes a noticeable delay.

I like having my data on one single sheet, BUT I get it. People want to see what games they have ONLY on the service (Gog, Steam, etc.) For this I suggest using the "Table" function (or "List" in Excel 2003 and lower). This gives you the ability to have every column act as a filter, so at any given moment you can quickly hide all the games that AREN'T listed under Gog.com in the specific "website" column... Or other meaningful info such as all the games you have under the FPS genre, or all the games never played, etc.

See here for ideas perhaps: http://www.gog.com/forum/general/magog_needs_your_help/page1

I do enjoy your completionator, but I would really like the GOG import function!

Merry Christmas!

Excel, Wordpad, etc... I too am on the neverending quest for something to catalog my games by genre, release, completed etc..

I too have made the double purchase quite a few times... Maybe I am missing it but is there a way to just download or copy and paste your game list from Steam / GOG.. Would be a great start anyways.

Also started to put games in backloggery but damn I would need a gram of dope to get that done. I shouldn't even being posting.. I should be playing one of the games I paid for years ago.. damn

Haha! Tell me about it. At one point it said I would need 170 x 24 hour days to complete all my games, but at the time I was playing games for only a few hours a week, so wouldn't complete my backlog until the year 2099... when I would be 131 years old.