You could add a new entry type "Duplicate" to your section with just an entries field "Use content from entry".

In your template you can now overwrite your `entry` variable with:

    {% set entry = entry.type == 'dupe' ? craft.entries.id(entry.contentFrom.first().id).first() : entry %}

You could also use the "Duplicate" entry type to link to the same entry / URL twice from your navigation:

    {% nav entry in entries %}
        {% if entry.type == 'dupe' %}
            {% set entry = craft.entries.id(entry.contentFrom.first().id).first() %}
        {% endif %}
        <a href="{{ entry.url }}">{{ entry.title }}</a>
    {% endnav %}

And in case someone "guesses" the URL, simply redirect them to the original entry:

    {% if entry.type == 'dupe' %}
        {% set entry = craft.entries.id(entry.contentFrom.first().id).first() %}
        {% redirect entry.url %}
    {% endif %}