reject duplicate arguments in #[interrupt] and #[exception]

This program was being accepted:

``` rust
 #[task(
    capacity = 1,
    capacity = 2,
    priority = 1,
    priority = 2,
)]
fn foo() {}
```

now it will trigger a compiler error

macros/src/syntax.rs

@@ -937,6 +937,13 @@ fn parse_args(input: ParseStream<'_>, accept_capacity: bool) -> parse::Result<Ta
         let ident_s = ident.to_string();
         match &*ident_s {
             "capacity" if accept_capacity => {
+                if capacity.is_some() {
+                    return Err(parse::Error::new(
+                        ident.span(),
+                        "argument appears more than once",
+                    ));
+                }
+
                 // #lit
                 let lit: LitInt = content.parse()?;
 
@@ -958,6 +965,13 @@ fn parse_args(input: ParseStream<'_>, accept_capacity: bool) -> parse::Result<Ta
                 capacity = Some(value as u8);
             }
             "priority" => {
+                if priority.is_some() {
+                    return Err(parse::Error::new(
+                        ident.span(),
+                        "argument appears more than once",
+                    ));
+                }
+
                 // #lit
                 let lit: LitInt = content.parse()?;