It’s documented:
The nature of keyword arguments makes it possible to specify the same argument more than once. For example, in the call
plot(x, y; options..., width=2)
it is possible that theoptions
structure also contains a value forwidth
. In such a case the rightmost occurrence takes precedence; in this example,width
is certain to have the value2
. However, explicitly specifying the same keyword argument multiple times, for exampleplot(x, y, width=2, width=3)
, is not allowed and results in a syntax error.