Display Items in Collection that Do Not Exist In Shopping Cart Shopify - shopify

Question

I'm trying to iterate over items in the shopping cart and generate an upsell div with products NOT in the cart. Below is the code I have so far, but I run into issues when adding two items to cart running the loop twice generating the html twice. Any ideas on how to resolve it? I'm stumped.
{% for item in cart.items %} // iterates over items in cart
{% if item.product.id == 4456879040188 %} // checks if product id matches in item in cart
<div class="upsell-pop" style="text-align:center; width: 100%;">
<h4>Frequently bought together</h4>
{% for prod in collections.upsell.products %} // iterates products in collection upsell
{% unless prod.handle contains "product-name" %} // shows only prods that do not contain url handle
<div>
<span class="upsell-title">{{ prod.title }}</span>
<span class="upsell-price">{{ prod.metafields["meta"]["promo"] }} {{ prod.price | money }}</span>
<img src="{{ prod.featured_image | img_url: '200x' }}" />
<a class="btn-product" href="{{prod.url}}">View Product</a>
</div>
{% endunless %}
{% endfor %}
</div>
{% endif %}
{% endfor %}
Another thought is to somehow check to see if product is NOT in cart items inside to replace the existing "unless" statement, but not sure how to code it.
{% unless cart.items exist then %} // I know this is not correct syntax
<div>
<span class="upsell-title">{{ prod.title }}</span>
<span class="upsell-price">{{ prod.metafields["meta"]["promo"] }} {{ prod.price | money }}</span>
<img src="{{ prod.featured_image | img_url: '200x' }}" />
<a class="btn-product" href="{{prod.url}}">View Product</a>
</div>
{% endunless %}

Answer 1

On my mind, there are two steps here.
First, capture your cart content to get string to compare to when you loop through your upsell collection. This could be something like that:
{%- capture cart_items -%}
{% for item in cart.items %}
{{ item.product.handle }}{% unless forloop.last %} , {% endunless %}
{% endfor %}
{%- endcapture -%}
Then loop through your collection while checking your string does not contain the handle of the current product at each iteration:
{% for product in collections['upsell'].products %}
{% unless cart_items contains product.handle %}
{{ product.title }}
{% endunless %}
{% endfor %}
Remarks:
Snippet 1 => In line_item (here called item because it's shorter to write) object you may access product object attributes: item.product.product_attr_needed
Snippet 2 => To access directly a collection object using its handle you must use collections + square brackets containing collection handle + attribute. Here 'upsell' is the handle of your upsell collection.
Not tested but this should work.
HTH