It is pretty difficult to transcribe a complex four part harmony vocal arrangement from a recording JamesH--much easier to write your own---and it is relatively easy to write something that has the sound or feel of another arrangement without it actually being the same.   For an arrangement to be the same, it literally has to be the same--note for note. Legally, you are entitled to use quite a lot, which is to say that a listener could identify different parts that are common to another arrangement without there being a copyright violation--

When the melody is traditional or folkloric, you're going to have a steep uphill battle proving that anything else in the arrangement is so unique that you can stake an exclusive claim to it. Basically, you'd have to show that someone Xeroxed your printed arrangement--or that you taught the arrangment to someone note for note(which is the same thing) and then they claimed it for their own.

The fact that something sounds like something else to you, doesn't mean that it is the same--and. particularly, it doesn't mean that someone has taken more than they are entitled to take--