fix: filter duplicate URLs from the sitemap XML

[boazpoolman]

What does it do?

It filters out duplicate URLs from your sitemap XML.

Why is it needed?

Because having duplicate URLs in your sitemap will get you penalized by Google.

How to test it?

Create multiple of the same URL and generate a sitemap.

[changeset-bot]

🦋 Changeset detected

Latest commit: 758e029

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
webtools-addon-sitemap	Patch

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.34%. Comparing base (b9c0ed1) to head (758e029).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #342      +/-   ##
==========================================
- Coverage   39.23%   38.34%   -0.89%     
==========================================
  Files           2        2              
  Lines         650      665      +15     
  Branches      165      163       -2     
==========================================
  Hits          255      255              
- Misses        316      328      +12     
- Partials       79       82       +3     

Flag	Coverage Δ
unit	38.34% <ø> (-0.89%)	⬇️

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