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I wasn't sure since as far as I know, we cannot repeat は or が (or am I wrong
about this?). I'm quite certain though that we can repeat の and に. But how
about を as in this sentence below?

私は封筒にきって **を** はるの **を** 忘れた?
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2016-10-28T08:20:55.937
40428
2016-10-29T00:05:32.283
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question
particle-を
Is it possible to use more than one を in a sentence?
Yes, を can be repeated in a sentence.  
And your sentence is perfectly correct:  
私は封筒に切手を貼るのを忘れた。  
I forgot to put a stamp on the envelope.

In this case the sentence is unambiguous because the corresponding verb
immediately follows を.
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2016-10-28T08:54:21.430
40429
2016-10-28T08:54:21.430
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answer
Multiple を may appear in one sentence when it's a [complex
sentence](http://examples.yourdictionary.com/complex-sentence-examples.html)
or compound sentence (i.e. has more then one clause).

> 私は封筒にきってをはるのを忘れた。

This is a typical complex sentence. 封筒に切手【きって】を貼【は】るの forms a _noun clause_
meaning "to put a stamp on the envelope". It serves as the object of the outer
main clause, 私は～を忘れた ("I forgot ～").

Similar question: [Is saying 「XはYは…」
acceptable?](https://japanese.stackexchange.com/q/19461/5010)
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2016-10-29T00:05:32.283
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2016-10-29T00:05:32.283
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answer