Sensei used twice in the same sentence?

In Lesson 1.7, to point out Tanaka is the teacher, we're given the phrase,

たなか　せんせい　が　せんせい　です

Tanaka sensei ga sensei desu.

From the content of the lesson, I was expecting to see something like

Tanaka-san ga sensei desu.

But instead, the word for teacher is used twice. Is sensei an alternative honorific in place of -san? And if so, isn't it weird to use it twice in the same sentence like this? If I'm understanding the lesson correctly, this basically says “Teacher Tanaka (is the one that) is the teacher.”

Well, I haven't even reached that lesson yet, but from my understanding, yes, it works as an honorific too.

So you know Tanaka-sensei is a teacher because of the honorific, but the sentence tells you he is the  teacher.

But I guess if you didn't know ehther he was a teacher you'd say “Tanaka-san wa sensei desu ka?”

Again, I'm just a beginner and I could be completely wrong on this.

こんにちは (Konnichiwa) Nicholas1024 and ssorgatem,

As ssorgatem said, せんせい (sensei) can also be used as an honorific title. As a noun, it means “teacher,” but when you add it after someone's name, it politely refers to them using the title of “teacher.” In English, this usually gets translated as “Mr.” “Mrs." etc, but literally, たなか　せんせい　が　せんせい　です (Tanaka sensei ga sensei desu) would mean “Teacher Tanaka is (the one that is) the teacher.”

You could also say たなか　さん　が　せんせい　です (Tanaka-san ga sensei desu), but by addressing Tanaka using the title of his profession (i.e. a teacher)  instead of さん (san), we can show a lot more respect. 

Think of how you might talk about a teacher in English. You wouldn't say “Smith is my teacher," but you would say “Mr. Smith is my teacher," right? It sounds a lot more respectful. 

Note that せんせい (sensei) can also be used to refer to other professionals, like doctors. So you could also use たなか　せんせい　(Tanaka sensei) to say “Dr. Tanaka." Don't worry, you'll be able to tell which is which from the context! 

I hope this helped! Please let me know if you have any other questions.

べんきょう　を　がんばって　ください！ (Benkyō o ganbatte kudasai!)