Newsletter/Blog Subscription Frequency

We have a team that wants to start sending per-post emails whenever one of their blog posts are published. We are currently sending a monthly digest to the list right now. Instead of just sending the per post emails to the list, we want to update the subscription center to allow people to opt in to whatever frequency they prefer. When typically asking frequency (monthly or weekly), should we allow a subscriber to select both or only one? The concern of allowing them to select both is that they would get duplicate content. But I am having a difficult time finding best practices or a preference center example that doesn't allow people to multi-select frequency. We're having a difficult time coming to a consensus on this so any insight, examples, or best practices would be appreciated.