Can the second "he spent" in this sentence be omitted? [closed]

I am certain that the time he spent in studying English is significantly more than that he spent in studying Japanese.

What English speakers would naturally do here to avoid repeating spend is not delete the second he spent but replace it with a form of the verb do, which for verbs plays the role that pronouns do for nouns.

I am certain that the time he spent in studying English is significantly more than that he did in studying Japanese.

Actually, that sentence is a little awkward; you can rearrange it to get a sentence that sounds better:

I am certain that he spent significantly more time in studying English than he did in studying Japanese.

(And you can delete all the occurrences of in in these sentences, although this doesn't address your question.)