I've been reading a lot of posts from users who can't have mono output to both speaker.
There is of post on hifiberry.com but I had to modify the conf file to make it work.
First, identify your card output with aplay -l
My card was identified as "card 2":
card 2: sndrpihifiberry [snd_rpi_hifiberry_dacplus], device 0: HiFiBerry DAC+ HiFi pcm512x-hifi-0 [HiFiBerry DAC+ HiFi pcm512x-hifi-0]
Then, write the following code to a file named /etc/asound.conf, and replace card 2 with the card you need.
If, like me, you are using squeezelite to play sound, you can now use the default alsa device (-o default when you launch squeezelite).
# Copy both input channels to output channel 0 (Left).
# Send nothing to output channel 1 (Right).