Topic marker は twice in one sentence?
The "Object" lesson has this sentence as translation for "I don't smoke":
わたしは タバコは すいません。
I am surprised because I thought you could only have one topic per sentence, so I was expecting タバコをすいません.
Assuming that this is intentional, does the second は convey a special emphasis? (Like, I don't smoke cigarettes but I do smoke other, um, controlled substances...)
There is really no grammar rule that says you can't use more than one は. In theory you can use an infinite number of は in your sentence and still remain correct, although that is a bad style. Using only 1 or 2 は is preferred.
But would it not make sense to use を, seeing as タバコ is the objecy of the sentence, not the subject?
Yes, you can think of it this way: "As for me, about tobacco, I don't smoke it."
は can be used to change topic or contrast between things. There's no rule there can only be one per sentence, but it depends on the context. You generally only need to use it once when introducing a new topic, and then the topic is understood and can even be dropped. In this case, assuming it was understood we were speaking about ourselves, we could drop the わたしは. But maybe there are multiple people in the conversation.
When は is being used as a topic marker, it can replace the を or が. There are other cases を can be dropped, you will see later, such as when using the verb する to turn nouns into verbs.
Keith and Harmony, thank you for explaining! I understand this better now.
Still, since there is no context within the exercise, it seems odd that they do not allow を here. But at least that gave me a chance to learn more about topic markers...
There is no reason it can't be を and there are probably many times when even native speakers use は when they should use を in a sentence like this. But the meaning is the same either way.
There are several variations on this construction, the most common being to leave out the は after わたし.
わたし、タバコは吸いません。 The implication, if any is intended at all, is that I might smoke a pipe or a cigar, but I don't smoke cigarettes.
わたしはタバコを吸いません。 The implication here, if any is intended at all, is that I don't know about anyone else, but I myself don't smoke cigarettes.
わたしはタバコは吸いません。 This is perfectly ordinary colloquial Japanese which would be understood to have the same implications as the other two sentences.