Duplicated headings in nested blocks

[percevalw]

Describe the bug

Nesting a docstring in another block (e.g., tab or admonition) produces duplicated headings.

tests/fixtures/headings.py

"""
Foo
===

### Bar

###### Baz
"""

index.md

Top.[^aaa]

=== "Tab A"

    ::: tests.fixtures.headings

In this case, the "Foo" header will appear twice in the produced html, both at the right position and after the inserted docstring :

<p>Top.[^aaa]</p>
  <div class="tabbed-set tabbed-alternate" data-tabs="1:1"><input checked="checked" id="__tabbed_1_1" name="__tabbed_1" type="radio" /><div class="tabbed-labels"><label for="__tabbed_1_1">Tab A</label></div>
  <div class="tabbed-content">
  <div class="tabbed-block">
  <div class="mkdocstrings">

  <div class="doc doc-object doc-module">



  <a id="tests.fixtures.headings"></a>
    <div class="doc doc-contents first">

        <h2 id="tests.fixtures.headings--foo">Foo</h2>
  <h4 id="tests.fixtures.headings--bar">Bar</h4>
  <h6 id="tests.fixtures.headings--baz">Baz</h6>

    </div>

  </div><h2 data-role="module" id="tests.fixtures.headings"></h2>
  <h2 id="tests.fixtures.headings--foo">Foo</h2>
  <h4 id="tests.fixtures.headings--bar">Bar</h4>
  <h6 id="tests.fixtures.headings--baz">Baz</h6>
  </div>
  </div>
  </div>
  </div>

Expected behavior

Headings from docstrings should not be duplicated.