Is it possible to use more than one を in a sentence?


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?




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.




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.

