It is often seen that the sentence *it is seen* is considered wrong but if we study voice then we will come to know *that that sentence* is written in the passive voice in which the object is hidden and the sentence is the passive voice of present indefinite sentence. Now someone can ask why that and that is repeated twice. Then my answer is:
Direct speech:
We will come to know, "This sentence is written in the passive voice."
Indirect Speech:
We will come to know that that sentence is written in the passive voice.
Now, you may perhaps ask why the tense is not changed then my answer is that is because the reported verb is in the future tense.
