You can decide what happens when someone already enrolled in your campaign texts your keyword a second time. For example, if you are running a contest or giving a coupon, you might not want an individual to enroll more than once.
Prevent Multiple Enrollments
By default, your campaign is set to start the campaign message flow again when someone texts your keyword for the second time. You can prevent multiple enrollments by enabling the Already Enrolled Message option:
1. Click the Edit Campaign Options button from the campaign editor.
2. Click Do not restart campaign. Send an auto-response message instead option to send a personalized auto-response message.
3. Customize the message to send if someone attempts to enroll more than once. 
4. Click Save Campaign Options.