Sometimes, if I approach a friendly gym which is currently under attack, and try to put an extra defender into the gym, I will be unable to do so, and the game will tell me "The gym is under attack".

However, sometimes when I am battling an enemy gym, i see someone walk past with a phone in their hand, and when I finish my current battle, I notice that they have put an extra defender into the gym while I was battling.

1. Is it supposed to be possible to put an extra defender in a gym while an enemy is battling that gym?
2. If so how?
3. If not, is there any way I can prevent that from happening while I am fighting gyms

Sometimes, if I approach a friendly gym which is currently under attack, and try to put an extra defender into the gym, I will be unable to do so, and the game will tell me "The gym is under attack".

However, sometimes when I am battling an enemy gym, i see someone walk past with a phone in their hand, and when I finish my current battle, I notice that they have put an extra defender into the gym while I was battling.

1. Is it supposed to be possible to put an extra defender in a gym while an enemy is battling that gym?
2. If so how?
3. If not, is there any way I can prevent that from happening while I am fighting gyms?