Learn English – Is it the formal writing to use double adverbs like “very very” or “really really”

"Phrasal verbs" are very common in English and have been around for a long time. Some are very old, some are recent inventions. Each has its own history and usage.

• Some of them are entirely acceptable in all registers:

  ^okAristotle refers to the effect of ethos and pathos on an audience.
  ^okI'm pretty sure that SOB was referring to me when he said that.

• Some are better used only in conversational registers:

  ^okPaul's always sounding off about how immigrants should be sent back where they come from.
  ^noIn his closing speech to the 20th Congress of the Soviet Union First Secretary Nikita Khrushchev shocked the delegates by sounding off for four hours on Stalin's reprehensible influence on foreign policy, the conduct of the war, agriculture, and civil society.

• And some are found almost exclusively in formal registers.

  ^okCopyright protection subsists in original works of authorship fixed in any tangible medium of expression.
  ^noJohn's folks subsist in the Ozarks.

There's no way of predicting which ones work where; you have to learn each one independently.

As John says, "very much" modifies a verb phrase. It means the same as the more casual "a lot" or "a whole lot". It's most natural position is after the verb phrase that it modifies, just like "a lot", but unlike "a lot", it can also go before the verb phrase that it modifies or after the main verb of that phrase.

When "very much" modifies a preceding complement construction, there is often a distressing ambiguity about whether it modifies the main verb phrase or the complement verb phrase: "I prefer not to eat snails very much" could mean either that your preference for not eating snails is extreme, or that snails are okay if indulged in only occasionally. That ambiguity is resolved by moving "very much" to a position either immediately before or immediately after "prefer", because then "very much" can only modify the entire verb phrase with "prefer".

On the other hand, "a lot" cannot be disambiguated in this way, since it has to go after the verb phrase it modifies. I think that this sort of disambiguation is mainly what "very much" is useful for, at least in casual conversation.

When "very much" comes before the verb phrase it modifies, it follows auxiliary verbs or other adverbs:

I have always very much preferred fish.
??I have very much always preferred fish.
*I very much have always preferred fish.