Why are these two songs nearly identical for 20 whole seconds? Could there be a theoretical explanation?

Question

Disclaimer: this question is not about copyright. I am trying to understand why the voice melodies of the two songs are so similar. I thought the chord progression could be classical, and that there could be a theoretical explanation for the melody.

Some time ago I noticed that the second opening theme of the anime "Your lie in April" has a part of roughly 20 seconds that is nearly identical to another part of the famous japanese opening of Dragon ball GT ("Dan dan kokorohitareteku..."). The chords are essentially the same, and the voice notes are almost the same. Here are two links to Youtube videos that point to the correct minute (from those spots, the relevant part lasts roughly 20 seconds):

Your lie in April (2nd opening theme) 0:45 - 1:04

Dragon ball GT (opening theme) 0:25 - 0:47

The chord progression (transposed in D major (?)) is:

G A F#m Bm(7) Em F#m Bm D(7)

(with a variant that starts with Em instead of G in the second song; I am not sure about the 7's, hence the parentheses). This pattern is repeated two times in "Dragon ball GT" and almost two times in "Your lie in April". I guess there is no need of saying, but the second song is half tone above the first one.

I am reasonably sure the first just copied from the second (because of the melody, not just the chord progression), but I also thought that there could be a small chance that this is not the case. In particular, I have two questions to ask:

1. Is the chord progression something recurrent/classical, that maybe both songs took from somewhere else? (For this, someone pointed me to this famous progression, although this explains only the first 4 chords).
2. Given the chord progression, do you think the notes the voice can/should follow are in a sense "forced by", or a natural consequence of the underlying chords? Or instead do you think there is enough freedom in the voice melody to conclude that one took inspiration from the other beyond any reasonable doubt?

Explanation to the second question: the voice melody in the first song follows the chords quite strictly; I tried to sing the same lyrics with different notes that still match the notes of the underlying chords, but I did not find any natural alternative without unpleasant jumps between the notes. Also, basically the melody is simply descending for most of the time. This makes me think that maybe the melodies matching in the two songs could be not that strange after all.

I have a basic understanding of how notes and chords work, but I am not a musician, and for both questions I think it is needed some experience and knowledge. Also, I am new to this site and I hope the question suits the site (as you can see, mine is a theoretical interest, even though motivated by anime openings :) ).

Answer 1

For most people, musicians included, it's actually really difficult to intuit how "similar" two melodies are by ear; by "difficult", I mean that usually listeners will be surprised upon a second listen or a closer examination/analysis. Often times, certain things that to your brain sounded "similar" might have differing notes or rhythms. That's why generally if I want to compare how similar two songs are, I'll go to the trouble of actually transcribing the relevant parts of the music. If that's not possible, find some good sheet music for them.

The question as it stands is pretty subjective; I don't think there's much to do except speculate on whether or not one song was inspired by another. I'll answer in the more general case. I am not a lawyer, but I do usually like to point out certain things about what "similarity" means from a musical property standpoint:

First of all, from a legal perspective, chord progressions aren't part of the creative identity of a piece of music. The fact that two songs have the same chord progression isn't really evidence that one song is copying another, but it does set them in the same environment and can sometimes influence melodies to be written with more apparent similarities. According to how courts have historically interpreted the relevant laws, a song is only its lyrics and melody. (That doesn't mean they've been terribly consistent on that stance in recent times, but it's a good starting perspective.)

Second of all, when you isolate the patterns in music that you consider to be similar, it's also worth considering where that musical concept came from originally. If the portion of the melody you think is similar is actually a very common idiom in early blues or rock music, it's actually a lot more likely that the two songs drew influence from their common body of stylistically-related music. Mariah Carey has almost certainly sang some riffs that Christina Aguilera has sung identically. Are they copying each other? No, they're probably both emulating the vocal style that they prefer. You could call this unoriginal, but remember that no one person or record label owns or invented a genre... for good reason.

Third of all, even if you can't really draw a connection between the similarity and a historical musical idiom, consider how unique the pattern itself is. Prince's "Raspberry Beret" has a chorus melody that contains the same notes as Joe Dassin's "Les Champs-Élysées" - and oh là là, they even rhyme with each other and both use French words. Clear plagiarism? In reality, when you get right down to it, this melodic idea is a simple 3-2-1 scale degree walkdown in a major key with one of the most basic variations on it possible. Is it fair to say that either artist created that musical idea for the other to steal? Heck, Three Blind Mice was written in 1609 by Thomas Ravencroft, so is it fair to credit him as a songwriter? And Ravencroft surely is not the originator of the melodic idea 3-2-1, a musical concept as old as melody itself. The point I'm trying to make here is that one should avoid focusing too hard on very basic musical elements to find similarity. There are some things too old and too fundamental to be owned.

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^{One more time, just to be clear: I have no legal expertise on intellectual property law and you should not trust the internet for such advice, but I understand musical history and theory enough to be able to interpret meanings in similarities most of the time. These are my opinions, not facts.}

Answer 2

As a composer, I have sometimes felt that some parts of the music I compose sounds similar in flavour to something I have heard before, but often I am unable to even figure out what that was. The point is that it is undeniable that previously encountered music indeed influences a composer's musical output, but it's often also unconscious, so it's not so easy to tell whether it was intentionally plagiarized or not.

Others have already pointed out that this kind of melodic contour is quite common, and there are a limited number of chord progressions that go well with it, so... It's not unusual for multiple people to come up with similar chord progressions..

I think one point that should be emphasized is that typically the melody comes first, and the chords follow naturally, so I don't agree with the implicit assumption in your question (2) that the issue is about what melodies fit the chord progression, since it should be the other way around!