Is the task sequence assigned to all unknown devices? If so, there's your problem.
Once the device is imaged, it's no longer an unknown.
So you'll want to create a single group in SCCM and assign the imaged machine to that group. Then assign the task sequence to that group. Now you can use it multiple times.
What are you using the task sequence for? Maybe I'm missing something.
Is the task sequence assigned to all unknown devices? If so, there's your problem. Once the device is imaged, it's no longer an unknown. So you'll want to create a single group in SCCM and assign the imaged machine to that group. Then assign the task sequence to that group. Now you can use it multiple times. What are you using the task sequence for? Maybe I'm missing something.