I just started listening to Rachmaninoff's variations on a theme of Corelli, and it's the theme from Liszt's Spanish Rhapsody!What's going on!?
For god's sake, how many original melodies did liszt ever manage to write?
Furthermore, there's nothing wrong with using melodies which already exist. Composers do it all the time, either quoting other melodies and works, or writing pieces completely based on older melodies.