I confirm this issue, started to happen after recent launcher update.
After more testing, it seems like this issue does not occur when instances are launched from installations with different settings.
Workaround for now: Duplicate the installation the user wants to launch multiple of, then launch one from each installation.
Also, it seems like changing the installation settings after launching one instance, then launching another works too (as said on previous comment).
Differences installation settings that prevents this error:
change in the 'version' setting. Even same MC versions work as long as it's a different version option(Mods).
Change in the installation name. However, changing the icon does not work.
ANY change to the VM argument setting; Even adding spaces works.
It can be noted that reverting/setting to the setting the first instance was launched with, then trying to launch causes this error to appear again.
DarkMatter3481
I confirm this issue, started to happen after recent launcher update.
After more testing, it seems like this issue does not occur when instances are launched from installations with different settings.
Workaround for now: Duplicate the installation the user wants to launch multiple of, then launch one from each installation.
Also, it seems like changing the installation settings after launching one instance, then launching another works too (as said on previous comment).
Differences installation settings that prevents this error:
change in the 'version' setting. Even same MC versions work as long as it's a different version option(Mods).
Change in the installation name. However, changing the icon does not work.
ANY change to the VM argument setting; Even adding spaces works.
It can be noted that reverting/setting to the setting the first instance was launched with, then trying to launch causes this error to appear again.