Can I obtain free transit visa twice?

Hello,
I'm Polish national and I will be flying from London - Benjing- Bali (Indonesia) and return.
My stop over to Bali is 9h in Benjing and 17h on the way back. The whole trip is 2 weeks.
I would like to see Benjing on the way to Bali and have a trip to see Great Wall on the way back.
Can I obtain it twice? Are the any restrictions how many times you are ask for free transit visa? Do you pay fees for it?
Thank you.