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Hi. Okay, that clearly is not supposed to happen.
I understand the behavior you have observed might appear frightening, especially if you already suspect others are getting access to information they should have no means to get access to. But I think we should assume it is a visual glitch, and that no compromise or attack is taking place. There have been reports of other similar duplication glitches since the Android 16 build, including for the lock screen, which clearly isn't an attack but just a glitch. I expect these glitches might be solved by the QPR1 update that is due soon. There is nothing in your situation or what is seen in that video that shows an actual attack happening.
Still, I wish we can debug the issue together to try to understand what is actually happening, as it shouldn't happen:
Is it correct that you have observed the behavior with two separate call screens opening on Piixel 6a and Pixel 9a devices, none other kind of GrapheneOS device?
Did all of your phones that got the duplicate call screens, and your girlfriend's phone too that got the duplicate call screen, did they all perhaps use the same carrier? Which carrier?
Did I understand correctly that you tried getting a new SIM card with a new phone number, and still saw the issue? Was that new SIM card perhaps also of the same carrier?
In the video there is an exclamation mark on cellular data connectivity. Why?
Are the Pixel phones officially supported in New Zealand with carriers there?
Did you enable any carrier overrides? Did you make any other configuration in SIM settings? Did you enable developer options? Did you change any developer options?
Have you installed any alternative phone call app?
How many user profiles do the affected phones have? Are the phone calls that get duplicate call screens made in the owner profile or in secondary user profiles?
Does the duplicate call screens appear only if you initiate the call, or only if you answer a call the other party initiated, or in both situations.
Can you make a new video where you clearly demonstrate no call screen is open, then show you dial the number or answering a call, so we can clearly see what happens right before and right when the bug happens?
Does the problem still happen if you disable VoWifi and VoLTE?
Does the problem still happen if you enable only VoLTE, and set carrier settings to "4G only", and also enable 2G restrictions?
Does the problem still happen if you enabled VoWifi, and then put your phone in airplane mode with Wifi enabled, so that the phone call will be done over Wifi instead of cellular?
Does the problem appear on a freshly factory reset phone, on which you have done nothing else than finish the first time setup wizard, plug in the SIM card and then immediately do or answer a phone call, with no additional settings done?
After doing the factory reset, can you show the boot hash that is showed on the boot screen with the yellow triangle?