How did my iphone pictures end up on another Iphone

The pictures that i took on my phone are not only in my photo albums but also in my father's photo album as well. How did that happen and how can i prevent that from happening again? There were over 100 pictures that transferred into his Iphone. He doesn't know how to do anything with his phone so i'm not sure how this happened.

Is he using your apple account?

Go to settings>icloud>photo stream. Then turn off "my photo stream." Shouldn't happen again.