The same event should not alert on every device
Aliases: notification mirroring · duplicate alerts · multi-device buzz
What it is
An iMessage arrives; the watch buzzes, the phone rings, and the same sentence pops in the computer’s corner—one event copied into three interruptions. Cross-device notification deduplication means a thing disturbs once, not once per signed-in device. Mirroring notifications to a Mac exists so someone already at the computer can see them; if mirroring means every device makes sound, mirroring is an amplifier. This entry is how many alert paths fire at arrival. It is not whether badges clear after a tap, and not how routing picks the device in use.
Why it happens
Every device registers with the same push channel. The default is deliver once per device. For the system that is reliable delivery: if one device is offline, another can still ring. For a person it is the event multiplied by the device count. Sound, banner, and haptic are three interrupt channels; three devices using them at once squares the interruption. There is no way to tell the devices that have not yet rung “I am already looking at one of these screens,” so the room sounds for several seconds. Unconditional “notification mirroring” skips dedup at the delivery layer and leaves filtering to per-device toggles—a settings page patching a wrong default. Dedup keys on an event id, not similar copy: two different mails may both alert; the push and the local banner for the same mail must not stack.
Where it stops holding
When devices are not in the same space (phone on the body, computer at the office), duplicate delivery is a backup, not noise; dedup would hide the device that is not nearby. Alarms, timers, and incoming calls succeed only if some device is answered; they may ring on several until one is picked up. A public or shared device should not receive mirrored private notifications at all; dedup is the wrong layer. A silent copy written only into Notification Center is not a repeated disturbance and may stay for later review.
Applying it
- Give each notification a cross-device event id and allow only one path to make sound or haptic inside a short window; other ends may write a silent Notification Center row.
- Do not, by default, replay the phone’s banner on every signed-in computer. Mirroring is “alert if the person is at that computer,” not “alert if signed in.”
- Offer a way to mute sound on other devices without wiping that device’s Notification Center history, so people are not forced to disable the whole class to get quiet.
- Verify with phone, watch, and computer awake on one Apple ID, then send a test message. Count sound and haptic paths; they should not exceed one. Silent records on the other two ends are allowed. Then close the computer and move the phone away: at least one path must still make sound, so dedup does not become silence everywhere.