Finding a proper adverb to end a sentence meaning unnecessary but not otherwise a problem

What sort of audience are you speaking to? Literary, technical, professional, casual? Would stylistic emphasis help you find the delivery that you're looking for (i.e. italicization or employing punction)?

The nature of your sentence strikes me as congenial/lighthearted. If that's true, maybe try a conversational approach:

• "Calling this function multiple times is unnecessary...but it's also (exceedingly/quite/totally) harmless."

Or

• "Calling this function multiple times is unnecessary — but — it's also (exceedingly/quite/totally) harmless."

The adverbs in parentheses are optional. They're there if you want to add a little cheek.

sounds like superfluous to me

su·per·flu·ous

/so͞oˈpərflo͞oəs/

adjective unnecessary, especially through being more than enough.

"the purchaser should avoid asking for superfluous information"

An adjective can work as well in your sentence.

Calling this function for multiple times is unnecessary but harmless either.

This sentence is incorrect, either would need a negative verb to make sense here:

Calling this function for multiple times is unnecessary, but it does not cause harm either. (not a very successful sentence)

Your second sentence is much better, nothing wrong with it. Other ways that would work just as fine would be:

Calling this function for multiple times is unnecessary but also harmless.

... is unnecessary but harmless at the same time.

I believe the entire idea can be captured concisely by a single word perhaps? How about "Calling this function multiple times is :" "extraneous" or "redundant" or as one other answer already suggested, "superfluous"

Calling this function multiple times is unnecessary but harmless.

The answer is simple. Your first version, shown above, is perfectly correct, logical and unambiguous.

Changing your original sentence is unnecessary and may even be harmful!

I would phrase like this:

It is not necessary to call this function multiple times, but it won't hurt anything.

or

Multiple calls to this function are unnecessary, but they won't hurt anything.

Additionally, if this is a relevant example, unnecessary function calls waste resources, memory, and processing time; so, it is harmful. You could thus rephrase simply as:

Multiple calls to this function are unnecessary.

And leave it here.

If you want to emphasize the contrast between the negative "redundant" and the positive (or at least neutral) "harmless" you could make it more explicit, reversing the order:

While calling the function more than once doesn't do any harm it is [entirely] redundant.

The advantage is that the main statement you want to make ("don't do that") ends the sentence.

Although given a choice I'd leave your original sentence, as others suggested as well.

Some answers say this, but to reiterate, your best bet in English (imo, and others here at least) is replacing the entire ending phrase with a single word. I say this with the background that it is technical writing and not a work of fiction, or similar where "flow" is more important than accuracy and conciseness. While I agree with the sentiment of other answers, I think the provided single word replacements are slightly off base. Instead of superfluous, extraneous, or redundant, try inconsequential.

"Calling this function multiple times is inconsequential"

Granted this phrasing is aimed at myself, a native english speaker - you know your audience better than I. But I think this is the best way to simply state what you're attempting to.

You could simply state that that "This function is idempotent".

Idempotence (UK: /ˌɪdɛmˈpoʊtəns/,[1] US: /ˌaɪdəm-/)[2] is the property of certain operations in mathematics and computer science whereby they can be applied multiple times without changing the result beyond the initial application.

https://en.wikipedia.org/wiki/Idempotence