Steps to reproduce:
Set up a recurring task that starts multiple recurrences back from the current time.
Open the task list in the left-side menu.
View the agenda view.
Double click on the checkbox next to the recurring task to mark the last two occurrences as completed.
Result: the task listed under the recurring task is opened.
If you doubleclick the task, it is intended behaviour to open it in the new window.
I consider it clicking on one occurrence of a repeating task and then clicking on a different occurrence of the same task. I consider occurrences of repeating task series to be different tasks. Because of this, I don’t consider this double-clicking a task.
It definitely doesn’t feel like intuitive behaviour to me; it violates the principle of least surprise.
OK, we will try to change it but until we fix it, simply click slower.