[Fixed]-Iterating records from DB (first record repeats)

The problem is in your nav.html

{% for slug in paper.slug %}
    <a href="{% url 'detail' slug=paper.slug %}">{{ paper.title }}</a>
{% endfor %}

In that line your are looping over a string, you should put there something like:

{% for paper in paper_list %}
    <a href="{% url 'detail' slug=paper.slug %}">{{ paper.title }}</a>
{% endfor %}