There’s a phenomenon whereby the songs in a musical reflect one another. That is, you can hear the melodies of other songs in the songs. For example, in Sweeney Todd, the tune for “Pirelli’s Miracle Elixir” is very similar to that for “God, That’s Good”. Sondheim does this a lot, and I can quite often hear three or four different songs reflected in one song, like threads. I’m thinking there must be a name for it. Any help?
For Sondheim, the technical term is “lack of talent.”
There are also concepts like “leitmotif.”
“Variations on a theme?” See Webber. 
Hey! I happen to love Sondheim!
But “leitmotif” sounds like the sort of thing I mean.
ALW does it so often we call it “recycling.”
Sondheim is an anagram of hedonism. Just thought I’d point that out.