What causes "duplicate Todo header" in GitoxideLabs/gitoxide?

Remove duplicate `Todo` lines so the document contains exactly one.

duplicate Todo header

Error message

duplicate Todo header

What it means

The edit document must contain the `Todo` enrichment header at most once. `parse` tracks this with a `todo_seen` flag and throws this error when a second `Todo` line appears, because duplicate headers would make the enrichment state ambiguous.

Solutions

Remove duplicate `Todo` lines so the document contains exactly one.
Regenerate the document with `document()` rather than concatenating or re-appending headers.
When programmatically editing, check for an existing `Todo` header before inserting one.

Example fix

// before (duplicated)
Todo
Todo
pick abc123 message
// after
Todo
pick abc123 message

Defensive patterns

Strategy: validation

Validate before calling

fn todo_header_count(body: &[&[u8]]) -> usize {
    body.iter().filter(|l| *l == &TODO).count()
}

Try / catch

match parse(&edited) {
    Ok(doc) => doc,
    Err(e) if e.to_string().contains("duplicate Todo header") => {
        eprintln!("Edit document malformed; regenerating.");
        regenerate_and_parse(repo, old_id)
    }
    Err(e) => return Err(e),
}

Prevention

When appending headers programmatically, check for existing ones first.
Never concatenate two edited documents.
Regenerate with `document()` when in doubt about document structure.

When it happens

Trigger: Calling `parse` (from `apply_conflict_reporting` or the parse tests) on a document body that contains the `Todo` header line two or more times, where non-comment lines are compared literally against the `TODO` marker.

Common situations: A tool or script that appends enrichment headers without checking existing ones; a merge of two edited documents; manual duplication while editing the todo file.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

AI-assisted analysis of GitoxideLabs/gitoxide@e73179060b (2026-09-08). Data as JSON: /api/errors/c4ffb7cfdb1e9fc8. Report an issue: GitHub.

Appendix: source

Thrown at gix-tix/src/edit/reword.rs:379

    let remainder = parts.next().context("the enrichment headers are missing")?;
    let mut enrichment = crate::enrich::Headers::default();
    let mut todo_seen = false;
    let mut message_seen = false;
    let mut message_offset = None;
    let mut consumed = 0;
    for line in remainder.lines_with_terminator() {
        consumed += line.len();
        let line = trim_cr(line.strip_suffix(b"\n").unwrap_or(line));
        if line.is_empty() {
            message_offset = Some(consumed);
            break;
        }
        if line.starts_with(comment_char) {
            continue;
        }
        if line == TODO {
            if std::mem::replace(&mut todo_seen, true) {
                anyhow::bail!("duplicate Todo header");
            }
            enrichment.todo = true;
        } else if let Some(title) = line.strip_prefix(MESSAGE) {
            if std::mem::replace(&mut message_seen, true) {
                anyhow::bail!("duplicate Message header");
            }
            let title = title.trim();
            enrichment.message = (!title.is_empty()).then(|| title.into());
        } else {
            anyhow::bail!("unknown commit header: {}", line.as_bstr());
        }
    }
    let message_offset = message_offset.context("expected an empty line after the commit headers")?;
    let message = cleanup_message(
        remainder
            .get(message_offset..)
            .context("the commit message is missing")?,
        Some(comment_char),

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