multiple `#[pin]` attributes don't error

[BennoLossin]

The following code is accepted today:

#[pin_data]
struct Struct<T> {
    #[pin]
    #[pin]
    inner: T
}

I think we should error on the second #[pin] attribute. Complaining that the first exists already.

Reported-by: Mohamad Alsadhan mo@sdhn.cc
Link: #113 (comment)

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Please only take this issue if you're not familiar with pin-init yet.

[BennoLossin]

@mqqz let me know if you want a different email to be used in the report link.