songs with the same tunes

It's called a leitmotif. Extremely common in opera and classical music. You identify a character with a certain piece of music. Frollo's theme is the Hellfire chanting heard at the BEGINNING of Bells of Notre Dame. The rest of the song is original and completely different from the Hellfire score. Don't know what you're going on about.

Technically, yes and no. "Hellfire" intergrates parts of "Bells of Notre Dame" into the song and finishes it, yet it is it's own song. But remember that "Bells of Notre Dame" intergrates a part of "Out There" as well.

Yes, the chorus of Hellfire - "Like fire, Hellfire" etc - is the same melody as the choral theme that opens the movie. But it doesn't form any part of the melody of The Bells of Notre Dame, at least I don't consider it inseparable from that song. And as someone already mentioned the theme is used as a recurring motif, something not uncommon, hardly lazy.

SO what is wrong with that, and I am sure it is hard to write lyrics or music for a song even an original song

really? do u think dat de reason 4 de songs having de same tunes is cuz of laziness? *shaking my head*