<!DOCTYPE html>
<html lang="en" itemscope itemtype="https://schema.org/WebPage">
  <head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>{{ site.title }} / {{ page.title }}</title>

    {% include styles.liquid %}

    <link rel="icon" href="{{ site.baseurl }}/images/{{ site.icon }}" type="image/png">

    {% if site.analytics.id %}
    {% include analytics.liquid %}
    {% endif %}

    <!-- metadata -->
    {% seo %}
  </head>
  <body class="{{ layout.bodyclass }}" id="top_of_page">
{% if page.tocDepth %}
    <span id="toc-depth" data-toc-depth="{{ page.tocDepth }}"></span>
{% else %}
    <span id="toc-depth" data-toc-depth="h2,h3"></span>
{% endif %}

    {% include nav.html %}
    {{ content }}
    <div class="page-spacer"></div>
    {% include footer.html %}

    {% include end-of-page-styles.liquid %}
  </body>
</html>


Remove <title> tag from template because the SEO plugin generates it now.
<!DOCTYPE html>
<html lang="en" itemscope itemtype="https://schema.org/WebPage">
  <head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">

    {% include styles.liquid %}

    <link rel="icon" href="{{ site.baseurl }}/images/{{ site.icon }}" type="image/png">

    {% if site.analytics.id %}
    {% include analytics.liquid %}
    {% endif %}

    <!-- Metadata *and* <title> Tag generated by SEO Plugin. -->
    {% seo %}
  </head>
  <body class="{{ layout.bodyclass }}" id="top_of_page">
{% if page.tocDepth %}
    <span id="toc-depth" data-toc-depth="{{ page.tocDepth }}"></span>
{% else %}
    <span id="toc-depth" data-toc-depth="h2,h3"></span>
{% endif %}

    {% include nav.html %}
    {{ content }}
    <div class="page-spacer"></div>
    {% include footer.html %}

    {% include end-of-page-styles.liquid %}
  </body>
</html>