My bad! Yes, you are right. I got the issue. Whenver you upload the image of the package, it is also included as gallery image. After that if you upload 2 gallery images, then you will have total 3 gallery images. The component is designed this way. If you want to change it, then you can customize in the code. I hope that you know the place to change. You have to change it here:
templates/travelin/html/com_spbooking/package/default.php
Then the changes will be preserved even if you update SP Booking.