You’re right that the page should not contain two h1 elements.
Go to Header1 widget in your template and replace this:
<b:include name="super.title"/>
With:
<b:if cond='data:view.isSingleItem'>
<h2 class="title">
<a expr:href="https://stackoverflow.com/questions/43148424/data:blog.homepageUrl">
<data:title/>
</a>
</h2>
<b:else/>
<h1 class="title">
<a expr:href="https://stackoverflow.com/questions/43148424/data:blog.homepageUrl">
<data:title/>
</a>
</h1>
</b:if>
5
solved Add headers h1, h2, h3 (Contempo)