Hey @nerd
Many thanks for your interest and for willing to help, highly appreciated.
In both cases - with the problem and without problem - thd dump looks exactly the same: there is single event logged (1 followed by 0). With problem it triggers doubled action, without problem it doesn’t but the event looks the same.
Two times mute pressed with doubled action:
volumio@volumiostreamer:/usr/sbin$ sudo ./thd --dump /dev/input/event*
[sudo] password for volumio:
EV_KEY KEY_MUTE 1 /dev/input/event7
KEY_MUTE 1 command
EV_KEY KEY_MUTE 0 /dev/input/event7
KEY_MUTE 0 command
EV_KEY KEY_MUTE 1 /dev/input/event7
KEY_MUTE 1 command
EV_KEY KEY_MUTE 0 /dev/input/event7
KEY_MUTE 0 command
Two times mute pressed without doubled action:
volumio@volumiostreamer:/usr/sbin$ sudo ./thd --dump /dev/input/event*
[sudo] password for volumio:
EEV_KEY KEY_MUTE 1 /dev/input/event7
KEY_MUTE 1 command
EV_KEY KEY_MUTE 0 /dev/input/event7
KEY_MUTE 0 command
EV_KEY KEY_MUTE 1 /dev/input/event7
KEY_MUTE 1 command
EV_KEY KEY_MUTE 0 /dev/input/event7
KEY_MUTE 0 command
As I restarted Rii Mini (I am using only wireless command, not IR) and as always after restarting triggerhappy process manually - it works fine, my best guess is that it not remote controller side problem.
My exact system version (it is in Polish but I think it is enough):

Links to both logs - first with the problem (captured just after system boot and with mute button pressed there times: first press muted system, second made doubled action - unmuted and immediately muted, third time exactly as second time:
http://logs.volumio.org/volumio/KQcNDdz.html
Link to the log after other reboot when the triggerhappy process has been restarted after boot (the same three times mute pressed - this time without doubled action. I had just sequence: mute - unmute - mute.
http://logs.volumio.org/volumio/nOAQRxG.html
volumio@volumiostreamer:~$ cat /etc/triggerhappy/triggers.d/audio.conf
#VOLUMIO TRIGGERHAPPY CONFIGURATION FILE
#MUTE TOGGLE
KEY_MUTE 1 /usr/local/bin/volumio volume toggle
#VOLUME UP
KEY_VOLUMEUP 1 /usr/local/bin/volumio volume plus
#VOLUME DOWN
KEY_VOLUMEDOWN 1 /usr/local/bin/volumio volume minus
#PLAY PAUSE TOGGLE
KEY_PLAYPAUSE 1 /usr/local/bin/volumio toggle
#NEXT
KEY_NEXTSONG 1 /usr/local/bin/volumio next
#seek plus
KEY_RIGHT 1 /usr/local/bin/volumio seek plus
#PREVIOUS
KEY_PREVIOUSSONG 1 /usr/local/bin/volumio previous
#seek minus
KEY_LEFT 1 /usr/local/bin/volumio seek minus
#power_off
KEY_SLEEP 1 /etc/trigger.sh
#repeat
KEY_BACK 1 /usr/local/bin/volumio repeat
#enter
KEY_ENTER 1 /usr/local/bin/volumio toggle
#peppy_screensaver
KEY_COMPOSE 1 /data/INTERNAL/peppy.sh
volumio@volumiostreamer:~$
Yes I confirm - the same I have noticed on two remaining rpi5 volumio installation as well. If I am not mistaken on RPI I have one system version back. The problem I have noticed after migration to volumio 4.
Btw the doubled action is most likely not only limited to volume and mute controlling. I have noticed that for instant for KEY_NEXTSONG action. As the mute test is the easiest to reproduce I described it together with volume.
Since reporting this issue on Sunday, I am using work around which fix this behavior - via systemd I am calling the script which does systemctl restart triggerhappy.service after system boot.
Hope it helps.
Thank you!!!