Your question isn't very clear (to me...). Are you trying to get an amplifier with a gain of 2x or are you trying to rectify a signal to get a DC value?
For the op amp, an x2 gain is possible as long as your output doesn't swing near the +- rails. For a single supply circuit, you will need a bias circuit to slide the AC signal up such that it doesn't go negative with respect to ground. All of this is covered in Dave's op amp videos.
Or Google for 'op amp videos'.
If you want to create a DC level, use a bridge rectifier. You can amplify the signal before you rectify it and you can even rectify it with op amps:
http://www.site.uottawa.ca/~rhabash/ELG4135L8.pdf