What is another way to express something that is idempotent?

There is a level of ambiguity in this question. It it the physical act of making transactions that should not be repeated—or is it the charge being applied against the card (regardless of the number of transaction attempts) that should not be repeated?

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Multiple Card Swipes

The most idiomatic way of saying it would be:

A credit card transaction should not be repeated so that you don't get charged twice.

But that uses a three-word verbal phrase.

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Merriam-Webster defines the adjective unrepeated:

: not repeated

As such, the sentence could be phrased:

A credit card transaction should be unrepeated so that you don't get charged twice.

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Although unrepeated is an actually defined word that nobody would misunderstand, it doesn't sound as normal as not to be repeated.

If it's not a problem forming a word from the normal rules of syntax, you could take the prefix non and apply it to the more normal adjective repetitive:

A credit card transaction should be nonrepetitive so that you don't get charged twice.

While I don't find nonrepetitive in Merriam-Webster or Oxford (at least in their online version), it does exist in more minor dictionaries such as The Free Dictionary and Vocabulary.com.

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Multiple Bank Charges

To mirror the previous sense, the most idiomatic way of saying this would be:

A credit card transaction should not be accruable so that you don't get charged twice.

From Merriam-Webster's definition of the verb accrue, from which accruable is formed:

3 : to accumulate or be added periodically
// interest accrues on a daily basis

Various websites use the word nonaccrual; for instance, Investopedia defines nonaccrual loans.

As such, it's not a stretch to think that the single-word form of the adjective to use in this sense could be nonaccruable:

A credit card transaction should be nonaccruable so that you don't get charged twice.

Possibly not quite what you're looking for but the word "debounced" comes to mind for me. As a hardware tech, to me it means performing only one action from multiple repeats of an input within a defined period of time.

debounce(Verb)

To remove the small ripple of current that forms when a mechanical switch is pushed in an electrical circuit and makes a series of short contacts. https://www.definitions.net/definition/debounce

and

debouncing

Bouncing is the tendency of any two metal contacts in an electronic device to generate multiple signals as the contacts close or open; debouncing is any kind of hardware device or software that ensures that only a single signal will be acted upon for a single opening or closing of a contact.
https://whatis.techtarget.com/definition/debouncing

Accepted use seems to be specific to multiple closures of the electrical contacts in a switch being acted on only once within a (usually short) debounce time, but I think the meaning of the word can be expanded to apply to longer time periods and other inputs, and could be used in your application if you're happy to forge a new use for it.

Thus:

A credit card transaction should be debounced so that you don't get charged twice.

You never know it could catch on...