What I imagine with the sentence 「(私は) お昼ごはんを もう食べています」is that I've started to have lunch and now I'm eating. 「わたしは お昼ごはんを もう食べました」 means I've alredy finished having lunch and now I'm not eating. 「お昼ご飯を もうたべましたか？」; Have you alredy had a lunch? Hope my English is good and it helps you.