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 theoptionsstructure also contains a value forwidth. In such a case the rightmost occurrence takes precedence; in this example,widthis 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.