Get Entries within a Category Group with no duplicates in each category

Question

How would I loop through all categories in a group to get this scenario:

• Get 4 entries per category
• No duplicates across any of the categories, if it's in Cat 1, should not show in any other.
• Category info (title, URL, etc)

So here is a visual:

Category 1

• Entry 1
• Entry 2
• Entry 3
• Entry 4

Category 2

• Entry 1 (duplicate from Category 1, do not want here, so get 4 different)
• Entry 5
• Entry 6
• Entry 7
• Entry 8

Category 3

• Entry 2 (duplicate from Category 1, do not want here, so get 4 different)
• Entry 6 (duplicate from Category 2, do not want here, so get 4 different)
• Entry 9
• Entry 10
• Entry 11
• Entry 12

I hope this gives a decent visual of what I want.

Answer 1

I've tested this with some of my own content and believe it works to your spec. You need to keep a track of all entries that have been displayed so far. You then loop through each categories related entries and if the entry hasn't been shown in a previous category we add it to a new array within that category loop.

    // Set empty array to hold id's of all currently displayed entries
    {% set entries_displayed = [] %}

    // Loop over a category group
    {% for category in craft.categories.group( 'productBrand' ) %}
      <h2>{{ category.title }}</h2>

      // Get related entries to the category
      {% set related_entries = craft.entries.relatedTo( category ) %}

      // Set empty array to hold id's of entries we'll eventually output
      {% set entries_to_display = [] %}

      // Loop through related entries
      {% for entry in related_entries %}

        // If the entry hasn't been displayed
        {% if entry.id not in entries_displayed %}

          // We want to show the entry
          {% set entries_to_display = entries_to_display|merge([entry.id]) %}
        {% endif %}
      {% endfor %}

      // Finally grab the entries to display from the array of id's that haven't already been shown
      {% set entries = craft.entries.id( entries_to_display ).limit( 4 ) %}
      {% for entry in entries %}

        // Add the id to our "already displayed" array
        {% set entries_displayed = entries_displayed|merge([entry.id]) %}

        {{ entry.id }} - {{ entry.title }}<br>
      {% endfor %}
    {% endfor %}

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UPDATE:

Erik later discovered an error in my original answer where I was setting entries_displayed in the first for loop before they'd actually been output in the last for loop. I've updated the code to reflect his fix.

I had to move the entries_displayed array to the last for loop which displayed the entries. That way I knew exactly what entries had been displayed. If I didn't do that, it would assume more than 4 were shown per category.