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?
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.
Multiple を may appear in one sentence when it's a complex sentence 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?