Let me explain what is going on here:
<user-ids type="array">
<user-id type="integer">678004</user-id>
<user-id type="integer">681456</user-id>
<user-id type="integer">683185</user-id>
<user-id type="integer">681456</user-id>
</user-ids>
It means that 4 devices will receive this push notifications. User with ID 681456 has 2 devices. That's why you can see 2 same ids 681456 and 681456 here.
Look's like one of your receiver was subscribed on sender's device too.
You can check user's devices in admin panel and manage them - open Admin panel, Users module, edit user - you will see all user's devices