The application you are using ran into a problem.
If you let their support team know about this error, please give them a link to this web page as well!
This page loads multiple Elm scripts. It then puts the publicly exposed modules from each script on the global Elm object. The problem is that there is a name clash.
For example, script a.js may expose an Elm module named Main, and script b.js may expose an Elm module named Main. Elm cannot know which one to use! So they need to have unique names if you need them to be added to the global Elm object.
Maybe a duplicate script is getting loaded accidentally? Anyway, if you reproduce this error in dev mode, it will tell you the module name that is getting added more than once.
Hopefully that helps!