Since Agile plugin version 1.6.2, a new feature was added - Allow overlapping Sprints. As the name suggests, it allows you to have overlapping dates for more than one Sprints' duration.
Please go to Administration (on top) - Agile. At the bottom of the settings page, you could see the Allow overlapping sprints.
If the Allow overlapping sprints feature is , then when you create a Sprint and the dates of it appear to be overlapping with some other Sprint dates, as a result, this warning message will appear and you will be not able to create the second Sprint.
Then, if you the option, the second Sprint will be created even though the dates of the two Sprints are crossing.