When a template uses inheritance and if you want to print a block multiple times, use the block function:

<title>{% block title %}{% endblock %}</title>

<h1>{{ block('title') }}</h1>

{% block body %}{% endblock %}

The block function can also be used to display one block from another template:

{{ block('title', 'common_blocks.html') }}

Use the defined test to check if a block exists in the context of the current template:

{% if block('footer') is defined %}
{% endif %}

{% if block('footer', 'common_blocks.html') is defined %}
{% endif %}