When I am writing a common app essay can I talk about something that is in another area of my application?

You are allowed to write about whatever you want to in your college essay. One dude wrote about zucchini pickles and he got into Brown University so..

I personally would suggest you to not to because the common app essay is a chance for you to be able to show a part of yourself that is not already seen or known from your application. I would suggest you to write two essays, the first about your volunteering experience and the other about another essay idea you have and ask people you know which one is better. If you do write about the experience volunteering, try to write in a narrative style telling a story while also explaining the lessons you learned. Never write what you learned or how you changed as a person at the very end.

For me I wrote about being homeless in a creative way. I began with the story of going on stage in the musical Annie and recognizing in the moment of singing " hard-knock life" how my life paralleled with the orphans. I talk about my unique situation of being homeless yet preserving through in academics and extra curricular. I shift back to the stage and listening to the song "maybe". I had a moment of hope and possibility for my future. I end with the classic line that the sun'll come out tomorrow despite how grey the skies may be today.

Hope this helped!

Absolutely, if you have a strong story that had an impact on you then you should use it. Don't forget to paint a picture of the story and describe how you've changed. Try to avoid using a generic story, make sure it's unique to you. Like you mentioned, I wouldn't over explain your volunteer experience in general, because they will have seen it in your resume.