{% assign is_duplicate = false %}
{% for check_product in collections.[collection.handle].products %}
{% for product in collections.[collection.handle].products %}
{% unless product.id == check_product.id %} <-- Ensure you're not just checking the same product against itself-->
{% if product.title == check_product.title %}
{% assign is_duplicate = true %} <-- i.e. you've found a duplicate!
{% endif %}
{% endunless %}
{% endfor %}
{% endfor %}
{% if is_duplicate %}
Do something...
{% else %}
Do something different...
{% endif %}