In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: hci_sync: hci_cmd_sync_queue_once() return -EEXIST if exists

hci_cmd_sync_queue_once() needs to indicate whether a queue item was
added, so caller can know if callbacks are called, so it can avoid
leaking resources.

Change the function to return -EEXIST if queue item already exists.

Modify all callsites to handle that.