If, for example. I launch an instance, use it for 10 minutes, terminate it. and shortly after I start a new instance. Will I be charged twice?

You are billed separately for each time an instance is running. Charges stop when you terminate the instance, and start again when the new instance runs. The documentation says credits are deducted “as long as the instance is running” and you may get extra charges if you forget to stop it, and instances launched with gmsaas keep running until stopped or until the max duration is reached. See gmsaas Commands.

To be precise, you won’t be charged twice for the same minutes. You’ll be charged for the first 10-minute run, then again for the second instance’s runtime.