Once a customer has been invited into a campaign, they cannot re-enter that campaign until 6 months after they have concluded the campaign.
However, you can bypass this! To send them another invite and enter them into another campaign, you can delete them as a customer in the People tab, and then re-add them in the 'Stories' tab (Stories> 'Get a review'). This leaves asking customers to leave you a review again within 6 months up to your discretion.
That means if you accidentally send a review invite to a customer that is already in a campaign, nothing additional will happen. They'll continue in the campaign they were already in.
Why? This prevents accidentally sending repeat messages to the same customer and turning a great customer into an annoyed one.