When you upgrade iCloud storage the new storage replaces the old one. It does not include or contain it, it merely replaces it.
That is, you don't get 250GB (200+50) of storage. You just get 200 total.
Where exactly are you seeing these invoices? How do you know they are both for iCloud+?
You could have a .99c charge for something other than iCloud+.
Check your recent charges by clicking here ➜ If you see 'apple.com/bill' on your billing statement - Apple Support
If you recently upgraded, and your previous plan also renewed shortly after, you may be charged for both. In that case, you would need to contact Apple support for a refund. It should only happen once though.
Otherwise, you cannot get charged for 2 different iCloud plans on the same apple account. The new one automatically replaces the previous one.
If you changed Apple IDs but did not cancel iCloud on the old one, it will continue to charge you.
If you are the Family Organizer in a Family Sharing group, you will be charged for subscriptions started by other members.