Learn English – English saying similar/equivalent to: “When you give, you receive twice as much.”

The closest I can think of is

a gift horse

which references the saying

Don’t look a gift horse in the mouth.

Though it is not a precise match.

A horse’s teeth are often a good indication of the horse’s overall health, so when purchasing a horse it was important to look in its mouth to ensure the teeth looked healthy. But the proverb tells you not to do this with a gift horse: since it was freely given, it does not matter what its condition is. Even an unhealthy horse is fine when it’s free. (The proverb is also given as a rule of etiquette, since it would be rude to question the value of a gift freely given.)

But if this is similar enough, then you could use a gift horse to refer to something you got for free, and therefore did not care how useful or valuable it was since it cost you nothing. It should be noted that while don’t look a gift horse in the mouth is a common, well-known saying, the phrase a gift horse referencing it is not common. I think most readers familiar with the saying will get it, but some might not get it immediately and react with “what horse?” You could give them a nudge in the right direction by saying something like a proverbial gift horse to indicate that you’re talking about the proverb, that this is a horse you shouldn’t look in the mouth.

As an aside, a term very different in meaning is a Trojan horse, which was a gifted horse that very much should have been inspected. It was, of course, a large wooden horse rather than a real horse, but it was famously left outside Troy by the Greek armies that had beseiged it. It appeared to have been left as a sign of respect for their bravery in defending the city, and it was taken into Troy. Of course, it was actually a trick: Greek soldiers had hid inside it, and that night they snuck out of it, now inside the walls, and the sneak attack took Troy. Hence the phrase beware of Greeks bearing gifts.

The fact that this trick took the form of a gifted horse while the proverb about not inspecting gifts also refers to a gifted horse is a historical coincidence, but one that is sometimes used to make a point (for example, about the limits of etiquette, or about the hidden costs of what might appear to be free).

I'm afraid your two options are not equally good. :)

Although "as good as each other" makes grammatical sense, we rarely compare things like that. We usually say "equally X" to say that they're equivalent.

Intuitively, I think the two forms have different purposes.

When you say "X is as good as Y", the purpose is to use Y as a point of comparison to tell how good X is.

Mmm! This cake is as good as the one you made for my 40th birthday. That one was amazing.

A cheetah can run sixty miles an hour. That's as fast as a car!

If this is the purpose, then it isn't helpful to say that two things are "as X as each other". Take "The sea is as black as night." The purpose is to use night to show how black the sea is. Night is your point of reference; you learn how black the sea is by considering how black night is.

So if I said, "The sea and night are as black as each other," we have a problem. "Each other" means that you learn how black the sea is by looking at the night... and you learn how black the night is by looking at the sea. That's a circular comparison, and not very helpful!

But the purpose of the word "equally" is the purpose you want.

"Equally" doesn't say how good two things are. It only tells you that they're equivalent.

It affirms that neither is faster or slower than the other — that there is no point choosing either one on the grounds of how good it is.

(You can also convey this message by saying that X is "no worse than" or "no better than" Y.)

Although Ngrams are not always useful, they can give us hints. Compare the rate of "equally good" vs. "as good as each other" in this Ngram. "Equally good" occurs 100-1,500x as often over various decades, which is such a high ratio that "as good as each other" is shown as a flat line by comparison.*

_{* Keep in mind that Ngrams are useful but not always definitive. See this discussion on ELU. It certainly doesn't mean there are zero occurrences of "as good as each other", and it also doesn't include similar wordings. For example, the classic children's book Onion John, which is full of idiomatic American English, says this in chapter 18: "Onion John could no more disappear out of Serenity, everyone agreed, than the courthouse could get up and wander off into the night. One was as much a fixed and regular part of town as the other."}

Extra note

As a minor caveat, sometimes you will hear "as" used to mean "equally" to contrast with another comparative on the spur of the moment, and it will be stressed there. Here's an example:

— "Would you say Amy is friendlier than Matthew?"
— "Hmm... no, I'd say she's as friendly as Matthew. They're equally friendly."

If you want to say that she's not very friendly, you could say she's "only as friendly as Matthew" – but this makes Matthew the standard against which to measure Amy, consistent with the above.