Tweak duplicate fmt arg error

Author: estebank

src/libsyntax_ext/format.rs

@@ -172,7 +172,8 @@ fn parse_args<'a>(
             let e = p.parse_expr()?;
             if let Some(prev) = names.get(&name) {
                 ecx.struct_span_err(e.span, &format!("duplicate argument named `{}`", name))
-                    .span_note(args[*prev].span, "previously here")
+                    .span_label(args[*prev].span, "previously here")
+                    .span_label(e.span, "duplicate argument")
                     .emit();
                 continue;
             }

src/test/ui/if/ifmt-bad-arg.stderr

@@ -138,13 +138,9 @@ error: duplicate argument named `foo`
   --> $DIR/ifmt-bad-arg.rs:40:33
    |
 LL |     format!("{foo}", foo=1, foo=2);
-   |                                 ^
-   |
-note: previously here
-  --> $DIR/ifmt-bad-arg.rs:40:26
-   |
-LL |     format!("{foo}", foo=1, foo=2);
-   |                          ^
+   |                          -      ^ duplicate argument
+   |                          |
+   |                          previously here
 
 error: positional arguments cannot follow named arguments
   --> $DIR/ifmt-bad-arg.rs:41:35