Syntaxerror: Failed to execute 'appendChild': Identifier 'ffCalc' has already been declared

[JimmyvanDelft]

Lately I've stumbled upon this error in my JS console, it occurs on pageload:

It does not seem to break anything, but would be nice to not have it ;)

It does not occur 100% of the time, mostly on first pageload. When refreshing it does not seem to occur anymore.

MW1.39 FF2.4.1