Hi Pradeep,
This could happen when customer has opened same email and performed operation- Subscribed , Unsubscribed and Subscribed.
1) User subscribed himself/herself or user is by default subscribed. That is when you will get first subscription.
2) User performed unsubscription by clicking on link provided in same email.
3) User performed subscription activity again via email received which redirects to subscription webpage.
Thanks.