@Bajurnik That does suck and one can only hope that the app author made a honest mistake and is not actively out scamming people. Because of the IOS label on your post, I can only assume that the app in question is from Apples app store, so my suggestions below reflect that.
1. Contact the author of the app, explain what happened and request a refund.
2. If you don't have any luck contacting the author or if the author won't refund, contact Apple about the charge and see if they will refund you the purchase.
3. If neither of the above work, gather up all your evidence including your attempts to solve this with the author and Apple and then contact your credit card company to dispute the charges.
If I'm incorrect in my assumption that the app in question didn't come from the app store but rather Google play then the following links may help you out.
https://support.google.com/googleplay/answer/2851610?hl=en&ref_topic=3364671&sjid=110203305781162144...
https://support.google.com/googleplay/answer/1050566?hl=en&ref_topic=3364671&sjid=110203305781162144...
