Why do I see the insert multiple times although capping is defined as once per session?

When you are testing a specific insert, you see the eye icon present in your app, and the insert will be displayed regardless of audience and capping settings.

Same is true in general for when your device is connected to Insert ('marketer mode') - in this mode you see all active inserts on your device, regarding of audience and capping rules.

To exit test mode, you need to disconnect your device from paired mode as specified in this article.