Default layouts are not working with `gatsby-theme-mdx-deck`

Description

// gatsby-config.js
module.exports = {
  plugins: [
    {
      resolve: 'gatsby-theme-mdx-deck',
      options: {
        contentPath: './src/decks',
        basePath: '/',
      }
    },
    {
      resolve: `gatsby-source-filesystem`,
      options: {
        name: `decks`,
        path: `${__dirname}/src/decks/`,
      },
    },
    {
      resolve: `gatsby-plugin-page-creator`,
      options: {
        path: `${__dirname}/src/decks`,
      },
    },
    {
      resolve: `gatsby-plugin-mdx`,
      options: {
        defaultLayouts: {
          default: require.resolve(`./src/components/Layout.js`),
        }
      }
    }
  ]
}

// Layout.js
import React from 'react'
import HeaderButton from './HeaderButton'

export default ({ children }) => (
  <>
    <HeaderButton />

    { children }
  </>
)

Syntax Error: SyntaxError: unknown: Identifier '_frontmatter' has already been declared (111:13)

  109 | MDXContent.isMDXComponent = true;
  110 |
> 111 | export const _frontmatter = {};
      |              ^
  112 |

Contributor guide

Tech stack

javascriptreact

Domain

frontend

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

2

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

1-3 hours

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

stale

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

Gatsby basicsMDX knowledge

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

50

Research direction

Investigate how the gatsby theme mdx deck default layout integration handles MDX content. The error indicates a duplicate frontmatter export in compiled MDX. Look at the plugin's source code (e.g., gatsby config or gatsby node files) to see how it generates MDX components and ensure default layouts are applied correctly without conflicting frontmatter declarations. The example repo provides a reproduction case.