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I understand only one song is playing repeatedly in your playlist. I will be happy to provide guidance for this behavior.
Have you had a chance to see if the Repeat option is enabled for that song? You can see how to review this setting from the following guide: Play, repeat, and shuffle music
Specifically, the following section may prove helpful:
Open the Now Playing screen and scroll down.
Let me know if the behavior persists after ensuring your Repeat setting is not enabled.