Trouble is, 'can' has two slightly different meanings: 'would you mind doing...' and 'are you able to do...'. 'Can you offer duplicates' as in asking GOG to offer duplicates woudn't be the opposite of 'Can't you offer duplicates', as 'can you not' would be the opposite of 'can you', rather than 'can not you'='can't you'. 'Can not you' is nonsense in modern English, but 'can't you' is used as a slightly sarcastic way of asking whether the reason something obvious is not being done is that it isn't possible to do (ie using 'can' in the 'capable of doing' meaning).

This is completely surmisal and I have no qualifications in this matter, by the way.

check please! I'm outta here! :P

You brought this on yourself ;-)

That's what Vader told the Alderaans!

Side note: Two Alderaan references in one day here. What's the record? :P

That's the negative still. "Can't you tell him?" is how you'd phrase that or "can you not tell him" but "can't you not tell him" only makes sense if the context suggests that the double negative is acceptable. In general it's not likely to be accepted as grammatical by native speakers because it's still a double negative.
This is collectively too much discussion of this, but the form the OP would be using is "can you not offer duplicates."

Wouldnt work, I know of at least 2 users here that own the entire catalog.

It would be like the old joke about breaking the internet by typing google into google

First, just for Hedwards: Can't you not post nonsense about the grammar?

Now, for everyone.

Yes, there's a certain issue that can't and can can mean the same thing:

"Can you smile a little more?"

"Can't you smile a little more?"

So, what's the difference? Both are requests to smile a little more.

Can you is a more direct, less sarcastic request. Can you please bring me a beer.

The choice to use 'can't you' instead of 'can you', in usage, in my opinion, adds an element that the requestor is a little more annoyed, it adds an element of 'you should have done this, but you didn't and I want you to' to it.

"Can't you smile a little more" has more a tone of "you should have been smiling more and I'm annoyed you didn't".

It can add that bit of aggressive tone of "I expect this, and you aren't doing it". That's just one possible meaning, though. Another is asking, 'are you able and willing to overcome the obstacle to do that?"

"Can you bring me a beer" is asking for a beer. "Can't you bring me a beer" tends to mean "you didn't bring one when I expect you to, is it too stinking much to ask", or "are you unable to for some reason".

In this case, 'can you' would have been ok, but 'can't you' implies that's how it should be done, and that it's simply a lack of having considered the issue or willingness to bother to do it that's in the way.

"Can you add Perfect General II to gog.com" is a request to add it. "Can't you add any of the best old wargames" is an expression of frustration that you think they haven't added games they should have.

Having the frustration that they didn't bother to prevent duplicates when it seems to me they should have and I've lost the offering of non-duplicates because they didn't made this the more exasperated "can't you" phrase instead.

It' also asking, 'is there some obstacle preventing you from doing it', with the expectation there isn't any good reason.

It's my job to know these things I am an English teacher. None of your examples involves a double negative. "Can't you" is perfectly fine, but "can't you not" is something that's unlikely to be accepted as grammatical.

"Can't you not" isn't a construct that's likely to be acceptable grammar as it involves a double negative which in most dialects of English isn't acceptable.

Like I said, I think this is more discussion than is really appropriate, but if you're going to call me out personally, the least you could do is read my post and generally know what you're talking about.

A linguistics professor was lecturing to his class one day. "In English," he said, "A double negative forms a positive. In some languages, though, such as Russian, a double negative is still a negative. However, there is no language wherein a double positive can form a negative."

A voice from the back of the room piped up, 'Yeah, right.'

Are there no languages where a double positive, sarcasm aside, forms a negative?

The problem is that the language is commonly used outside the boundaries of "official grammar". As Craig234 pointed out, in the context of a question, "can you" and "can't you" actually mean the same thing, only with different inflections. Whether that is gramatically correct or not, you cannot deny that that is in fact how the language is used. That is the only the first part of the question though. The second part is the action which the first part refers to. In this case, the requested action is "not offering duplicates". As such, there is no double negative, because the two negatives involved appear in semantically separate sections of the sentence (alliteration FTW).

For the record, I am not an English teacher, nor is English my first language, and I just pulled the above explanation out of my ass. Still, it makes sense to me.

Being a descriptivist normally I would agree with you.

But, can't and can in this context don't quite mean the same thing. They approach it from opposite ends and while they're often used to communicate a similar idea, the former isn't normally used with a negative. It's already asking about the negative possibility, so adding a negative later on just confuses the issue.

"can't we get along" is something people would normally say, but "can't we not get along" isn't. "can't we not fight" is likewise rather strange and would require a certain amount of contextual build up to make sense. It's rather unusual to assume that we have to fight, so asking if it's possible to not fight would be rather silly.

In spoken language or language where there's more context, it probably would work, but as a title, it does represent a global error that renders the sentence itself ambiguous. Sarcasm and snark rarely work in titles unless the writer is well known for it or is writing in a publication where such snarkiness is to be expected.

I don't expect, nor do I desire, perfection but rationalizing what is clearly an error is kind of silly.

Anyways, I'm out of here, I have far better things to do with my time than to argue over such trivials.

hedwards may teach English, but he or she completely missed the humorous tone of my post to them.

Reading comprehension and grammar are two different things.

I don't think any reasonable person would miss the intent of the thread summary, 'can't you not offer duplicates', which is the same as 'can't you avoid offering duplicates', and any grammatical issues with it are trivial and unimportant.

That's from someone who is generally sympathetic to grammatical correctness.

There is nothing ambiguous about the title, and it does not contain sarcasm.

The first part - can't you - has been established to mean the same basic thing as 'can you', with a bit more expectation in the tone.

The second part - not offer duplicates - is negative simply because it's requesting the stopping of an activity instead of the performance of an activity.

Saying 'can't you not talk for two minutes' is the same thing as 'can't you be quiet for two minutes', with just a different style of statement, the former implying 'you seem unable to restrain yourself from talking'.

Imagine after a disaster - war or otherwise - former adversaries find themselves survivors. Some want to continue violence even then, but someone recognizing it's time for that to stop says 'can't we not attack each other even now when there are so few left?' The reason for the double negative there is to express the expectation that people should recognize the situation has changed and violence is now unreasonable. It makes sense.

And my asking somewhat exasperated, 'can't you not do that unreasonable thing' was as well.

I think 'can't you not offer duplicates' is more clear than 'can't you offer only games we don't have'.

Open another account click another box and gift it to your main account

That doesn't help - the issue is wanting to get a game I don't own already in the offer.