What the heck are Queued customers in a campaign?

So, ever since I’ve noticed that there are people with the “queued” status in sequences on a campaign, they have been receiving duplicate emails on the same email. What does queued mean and how do I prevent people from becoming queued?

I’m not sure how queued relates to duplicate emails being sent. But here is an explanation of queued contacts: https://help.infusionsoft.com/help/why-are-contacts-queued-in-my-campaign