How do I make a tween loop without it affecting the key frames?

I am trying to animate a character walking and talking at the same time. I want to make a loop of the head bobbing over and over while they're lip syncing to the audio. I made the head a graphic symbol and tried using tweens, but every time I try to copy and paste the tween over and over, it makes the graphic start all over again. How do I loop the tween without it affecting what is happening inside the graphic?