This feature ensures that a lead cannot enter or be in a campaign multiple times. This is useful for most campaigns though there are examples where you would want a lead to be able to enter into a campaign more than once (e.g. if a lead reschedules an appointment and needs to enter back into an appointment confirmation sequence)
Part 1: Allow Multiple Campaign Configuration
Step 1: Enter into the campaign that you wish to edit.
Step 2: Drop down the Campaign Configuration Menu. Your Allow Multiple button will be turned on by default. Toggle it off. Having it turned off will not allow you to add your leads to enter into the campaign more than once.