If you just close a deal and you are third part and one of your partners do business on your back whithout you having idea , what is good to do ?

If the contract between all parties was not made with this scenario in mind, then you need to strengthen your position on future deals. If your position is important and you clarify to both parties that they will loose your part for any future contracts, they would fear loosing you.

  This happens quite often for example in supplier contacts where multiple parties are involved. A good contract covers these concerns, but even with a good contract this scenario might happen. Only way is to realize all parties your importance for subsequent business in future.

Terminate the partners if it is your business.