Realizing Capital Gains

Scenario: I bought stock XYZ at 20 about 6 months ago. Recently I sold at 40, and then bought back at 38 (sell and rebuy occured on same day). I was expecting to have a realized gain from 20 to 40, and have a "new" position at 38. My online broker gains tracker (ameritrade) instead is showing a realized gain from 38-40, and an unrealized gain from 20 still. Is that correct? If so, how can I realize the gain while holding the stock?

Thanks.