Learn Italian: Il pericolo è imminente. Il veggente lo ripete spesso. - The danger is imminent. The clairvoyant repeats it often.

Here, lo is a "pro-clausal" pronoun. It stands in for the entire preceding idea (Il pericolo è imminente). While English sometimes drops the object in these cases (e.g., "The clairvoyant repeats often"), Italian transitive verbs like ripetere strongly prefer a direct object pronoun when referring back to a specific, already-mentioned fact.

Is lo always masculine singular when replacing a whole sentence?

Yes. Whenever you use a direct object pronoun to encapsulate an abstract idea, an action, or a whole clause rather than a specific noun, you must use the default masculine singular form, lo.

What is the origin of the word veggente, and does it change for gender?

Veggente comes from the verb vedere (to see). It is essentially a present participle used as a noun, literally meaning "the seeing one" or "seer". Because it ends in -ente, the word itself doesn't change based on gender. You rely entirely on the article to know if the person is male (il veggente) or female (la veggente).

Could the adverb spesso go before the verb, like in English?

It's much less natural. In Italian, adverbs of frequency like spesso typically follow the verb they modify (lo ripete spesso). Placing it before the verb sounds clunky and is generally avoided unless you are using a very specific poetic or highly emphatic structure.