I purchased the PBE and xanathars guide on the app and it is not giving the option to download. It takes me to the market place again and tells me I already own the content. 

Aside from this the app constantly crashes. Any advice?

I recently had the same issue.  I deleted the app then reinstalled it.  That seemed to fix it for me.

I did as well i had to purchase it through here to get acess to it so i had to pay for it twice

That's odd that you had to purchase it twice as it should be connected to your dndbeyond account once you purchase it.

It would say "content is already owned" in app. However it wouldn't load on any platform. 

Maybe we've been getting this issue because dndbeyond is working on consolidating the two apps.  https://www.dndbeyond.com/posts/965-the-all-new-d-d-beyond-app-is-coming