bent over barbell row

today the barbell clipped my right knee while doing the bent over barbell row,is this common? how to stop this from happening again?

It's somewhat common if you have bad form. Don't keep your hips too low or you're knees will stick out and don't put the bar too close to you or you'll whack your knees when you pull up. If you have long, gangly legs set up your feet slightly pointing out so your knees do the same (but not so much so that it compromises your form).

So are legs really bent?
Some knee bend if fine but not like your at the bottom of a squat.
Make sure your not moving the bar all over the place but in a nice groove the same for every rep that you feel the target area in your back the best.
Another option would be rowing from pins in a rack stopping every rep briefly on the pins then pulling again.