It is actually not a double charge.
Public mobile is a prepaid phone service provider. You pay your monthly fees upfront. Not refunds or credits. When changing plans there are two plans presented. They do let you know the best option to choose.
Change now results in automatic new charge. Or change on Renewal which will allow you to finish out your cycle and start new at the next payment; no money loss.
you can always try your luck and create a ticket through the chatbot and ask.
