This example shows that non-summary notifications are displayed even if a summary notification exists on some configurations.
To trigger this behavior, you must create some notifications and add them to a group. Then trigger a summary notification that has set an sound (ringtone, vibration) and a priority of high.
A notification that has set a group with setGroup(String).
A notification that has set a group with setGroup(String) and setGroupSummary(true).
I would expect Android to not display any notification but the summary notification. I set sound and priority to high to display the summary notification as heads-up notification.
I tested this behavior on Android 5.0 and 5.1 (with an emulator).