If you complete a repeating task, a copy will be created with a new due date. If the original task had subtasks, the new subtasks will have the same dates as the old ones. So I think what you’re seeing are the subtasks assigned to the first task and the subtasks assigned to the second task, which both have the same names and dates.
What I usually do is make the subtasks repeating but not the task. When the first round of subtasks are completed, they come back to life with the correct new dates. Then I change the due date of the task manually. In other words, the task is never completed, I just give it a new date whenever the subtasks are completed and reincarnated.
Note that you should not make both the task and subtasks repeating.
Here’s more information about repeating tasks: