On YesJapan and in the Japanese From Zero! book series in which YesJapan.com is based we do say that you can not use
more than once in a sentence.
This is a rule that is broken in a couple of cases:
1. If you are quoting another sentence. 2. If you are using
to stress a certain part of the sentence.
In the sentence
Kotoshi wa natsu yasumi o toru tsumori wa arimasen.
the speaker is stressing 2 that "this year" she has no intention of taking a summer break. This usage of
implies that maybe she normally goes, or perhaps this year is special.
I will say this about Japanese teaching methods. I am a big fan of teaching broad rules about something so that it sticks. Then later on you can teach about exceptions.
I guess this is similar to how you teach your children big concepts. Like you tell them to NEVER do drugs. But then later they find out that there are exceptions for when you are sick. The bottom line is that they know to be cautious about doing drugs.
The same thing applies here with using
twice in a sentence. It is totally possible for it to happen, but you just want to be aware of how it is used.