Can a parameter-name occur twice or more?

I.e., is

n;p=a,p=a,p=b:.

a valid contentLine?

It's syntactically valid, and I can't see anywhere its forbidden.

I ask because I want to know whether the decoding API I am writing
can assume that this case will not occur, and/or that if it does
it is invalid, and I can just use the first occurrence as the value,
or something like that.

Thanks,
Sam