Why does the one-time discount code show on multiple upcoming orders?

Discount codes are applied on the address profile and will appear on all queued orders under that address. However, if the code is configured to only apply to one charge the code is removed from the other orders after an applicable charge is processed.

This setup ensures that if changes are made to the order schedule the code is still applied to the next applicable order that processes.

It is recommended to check if you have enabled the Automatically remove discount codes when they expire, or their usage limit has been reached setting to ensure the code is automatically removed once it is used and allow a new code to be applied if necessary. 

This setting is found in under Storefront > Customer portal, in the Discount code expiration section. If you don't see this setting, it is already enabled on your store.