If you stay in the MTR pay area after 20minutes and you 同站出入, you need to pay $10.
You may take the MTR by one station and go out and you go back to your original station. Here is the fare:
Ngau Tau Kok to Kwun Tong: $3.8
Kwun Tong to Ngau Tau Kok: $3.8
Total: $7.6
It is more cheaper than 同站出入