Fix: Duplicate Content Entry Slug in Astro

What Causes This Error?

This error occurs when two or more content entries in the same collection have the same slug. Each entry must have a unique slug to generate unique URLs.

The Problem

src/content/blog/
├── post-one.md      # slug: "my-post" (from frontmatter)
└── my-post.md       # slug: "my-post" (from filename)
                     # ❌ Duplicate slug!

---
# File 1: src/content/blog/article-a.md
slug: "featured-post"
---

---
# File 2: src/content/blog/article-b.md
slug: "featured-post"  # ❌ Duplicate!
---

The Fix

Use Unique Slugs

---
# File 1: src/content/blog/article-a.md
slug: "featured-post-2023"
---

---
# File 2: src/content/blog/article-b.md
slug: "featured-post-2024"
---

Or Use Unique Filenames

src/content/blog/
├── featured-post-january.md   # slug: "featured-post-january"
└── featured-post-february.md  # slug: "featured-post-february"

Common Scenarios

Filename vs Frontmatter Conflict

# File: src/content/blog/hello-world.md
# Default slug from filename: "hello-world"

---
# If another file has this in frontmatter:
slug: "hello-world"  # Conflicts with the filename-based slug
---

Solution: Choose One Method

---
# Option 1: Use meaningful filenames, no slug in frontmatter
# File: hello-world.md (slug is "hello-world")
title: "Hello World"
---

---
# Option 2: Use IDs as filenames, slug in frontmatter
# File: 001.md
title: "Hello World"
slug: "hello-world"
---

Nested Directories

src/content/blog/
├── 2023/
│   └── review.md      # slug: "2023/review"
└── 2024/
    └── review.md      # slug: "2024/review" ✅ Different slugs

Finding Duplicates

# List all markdown files and their slugs
grep -r "^slug:" src/content/blog/ | sort

# Or check filenames
ls -la src/content/blog/

In Build Output

Error: Duplicate content entry slug "my-post" found in collection "blog".
Entry: src/content/blog/post-a.md
Entry: src/content/blog/post-b.md

Programmatic Slug Generation

// If generating content programmatically, ensure unique slugs
const posts = [
  { title: "Post 1", slug: "post-1" },
  { title: "Post 2", slug: "post-2" },
  // Ensure no duplicates before creating files
];

const slugs = new Set();
posts.forEach(post => {
  if (slugs.has(post.slug)) {
    throw new Error(`Duplicate slug: ${post.slug}`);
  }
  slugs.add(post.slug);
});

Using Dates for Uniqueness

---
# Include date in slug for uniqueness
title: "Weekly Update"
slug: "2024-01-15-weekly-update"
---

---
title: "Weekly Update"
slug: "2024-01-22-weekly-update"
---

Category/Type Prefixes

---
# Use category prefix
title: "Getting Started"
slug: "tutorial/getting-started"
---

---
title: "Getting Started"
slug: "guide/getting-started"  # Different category = unique
---

Migration: Handling Existing Duplicates

// Script to find and fix duplicates
import { glob } from 'glob';
import matter from 'gray-matter';
import fs from 'fs';

const files = await glob('src/content/blog/**/*.md');
const slugMap = new Map();

for (const file of files) {
  const content = fs.readFileSync(file, 'utf-8');
  const { data } = matter(content);
  const slug = data.slug || file.split('/').pop().replace('.md', '');

  if (slugMap.has(slug)) {
    console.log(`Duplicate: ${slug}`);
    console.log(`  - ${slugMap.get(slug)}`);
    console.log(`  - ${file}`);
  } else {
    slugMap.set(slug, file);
  }
}

Quick Checklist

• Each entry needs a unique slug
• Check both filenames and frontmatter slugs
• Use dates or categories for uniqueness
• Nested folders create different slug paths
• Run build to catch duplicates early