Step 1. Navigate to your campaign or the contacts page
Search for the contact and check the column to the left.
Step 2. Check if the contact is active
When a contact has a green dot next to their name, this means they are active in a campaign. As long as the contact does not have the green dot next to their name, they are not active in the campaign and can be added to a new campaign or re-added to the existing campaign.
More details on how to check if a contact is active.
Step 3. Re-Add contacts to the Campaign
Select all contacts you'd like to move
Click 'Add to Campaign'