When you log in to "manage feedback requests" all of the types of feedback are in one long list.
This list includes people who you have sent to once, people you have sent to twice, and people who have already left feedback.
Could you not have it so that once a customer has left feedback they are removed from the long list and added to a list called something like "feedback received"
This stops us running the risk of selecting the tick box of someone who has already left feedback and re sending.
Only two lists are needed - "pending feedback" and "feedback received"
Alternatively, if they have left feedback, maybe they could be colour coded to alert you better that simply a line of text mixed up in other lines of text?