Why do the Xiaomi Buds 5 Pro and the phone ring together when a call comes in?

For your reference, here are the reasons about both the phone and earbuds play sound at the same time：
1. Because iOS/Android phones will automatically select the ringtone play path according to whether mute is enabled or not. Without mute turned on, the sound is played on both the phone and earbuds during incoming call; with mute turned on, the sound is played on the earbuds only;
2. During the connection process of earbuds, some devices will not switch the audio path correctly. Although the earbuds is successfully paired and connected, when a call is received, the audio streams of the phone and earbuds are not be fully synchronized, causing both to play sound at the same time. You can try disconnecting the Bluetooth earbuds and reconnecting it.