Function detect_duplicate_wrapper_defs 

fn detect_duplicate_wrapper_defs(
    content: &str,
    accessor_producers: &HashMap<String, String>,
) -> Result<(), String>

Detect a second top-level name <- function definition of any wrapper function in the assembled R wrapper text.

This is the write-time counterpart to the compile-time check_s7_shortcut_collisions (in miniextendr-macros): it catches name collisions that span separate macro expansions and are therefore invisible to any single proc-macro invocation. The two motivating cases (#991):

1. An S7 fast-path shortcut <Class>_<method> colliding with a #[derive(ExternalPtr)] sidecar accessor <Class>_get_<field> / <Class>_set_<field> – the sidecar field names live in the MX_S7_SIDECAR_PROPS slice, which the impl-block macro can’t see.
2. Two #[miniextendr(s7)] impl blocks on the same type each emitting a <Class>_<method> shortcut – cross-impl-block, also invisible to a single expansion.

Without this check the second definition silently clobbers the first (last-write-wins in R), so a call dispatches to the wrong implementation.

accessor_producers maps a known sidecar-accessor name to a description of its producing derive; when a duplicate matches one we name the derive in the message, otherwise we report the generic “defined more than once” form.

Returns Err(message) naming the offending function on the first collision found; Ok(()) when every top-level definition name is unique.