If you are talking about a child redownloading apps, there is no setting preventing a child from re-downloading an already purchased app.
A more extreme solution completely bypassing Ask To Buy is proposed by klaritykhaos in this post:
How can I prevent my child from downloadi… - Apple Community
“As a workaround you can go to Settings> parental controls > itunes and app store purchases> turn off Installing apps. That way you will have full control of which apps are installed. This does work for redownloads.”
If you wish to make a recommendation to Apple about adding a feature to prevent redownloading of a previously approved app, go to —> Product Feedback - Apple