Add an error message for declaring a variable multiple times in the function header

[Boulder08]

I just spent some 15 minutes trying to find out why my function was not working properly, i.e. not respecting a change in value in the function call. I then found out the reason; I had declared the variable two times in the function header by mistake. I think we need an error message for this situation the same way you get an error message when calling the function with two identical variable names as parameters.