Add gatsby-remark-responsive-iframe to gatsby-plugin-mdx

Hi Sir,

I am trying to get responsive iframe working after embedding a youtube video. I try adding the following in gatsby-config.js

plugins: [
    { resolve: `gatsby-theme-document` },
    {
      resolve: `gatsby-plugin-mdx`,
      options: {
        extensions: [`.mdx`, `.md`],
        gatsbyRemarkPlugins: [`gatsby-remark-responsive-iframe`],
      },
    },
  ]

However, this is giving me an error when running gatsby develop:

Generating development JavaScript bundle failed

unknown: Identifier '_frontmatter' has already been declared (101:13)

   99 | MDXContent.isMDXComponent = true;
  100 | 
> 101 | export const _frontmatter = {};
      |              ^
  102 | 

File: src/gatsby-theme-document/logo.mdx:101:13

failed Building development bundle - 3.046s

Can anyone please shed some light as too why this is not working?

Thanks,
Andy