diff --git a/ghost/config-url-helpers/test/unit/utils/deduplicate-subdirectory.test.js b/ghost/config-url-helpers/test/unit/utils/deduplicate-subdirectory.test.js
new file mode 100644
index 0000000000..4eb62c3f70
--- /dev/null
+++ b/ghost/config-url-helpers/test/unit/utils/deduplicate-subdirectory.test.js
@@ -0,0 +1,139 @@
+// Switch these lines once there are useful utils
+// const testUtils = require('./utils');
+require('../../utils');
+
+const deduplicateSubdirectory = require('../../../lib/utils/deduplicate-subdirectory');
+
+describe('utils: deduplicateSubdirectory()', function () {
+    describe('with url', function () {
+        it('ignores rootUrl with no subdirectory', function () {
+            let url = 'http://example.com/my/my/path.png';
+
+            deduplicateSubdirectory(url, 'https://example.com')
+                .should.eql('http://example.com/my/my/path.png', 'without root trailing-slash');
+
+            deduplicateSubdirectory(url, 'https://example.com/')
+                .should.eql('http://example.com/my/my/path.png', 'with root trailing-slash');
+        });
+
+        it('deduplicates single directory', function () {
+            let url = 'http://example.com/subdir/subdir/my/path.png';
+
+            deduplicateSubdirectory(url, 'http://example.com/subdir')
+                .should.eql('http://example.com/subdir/my/path.png', 'without root trailing-slash');
+
+            deduplicateSubdirectory(url, 'http://example.com/subdir/')
+                .should.eql('http://example.com/subdir/my/path.png', 'with root trailing-slash');
+        });
+
+        it('deduplicates multiple directories', function () {
+            let url = 'http://example.com/my/subdir/my/subdir/my/path.png';
+
+            deduplicateSubdirectory(url, 'http://example.com/my/subdir')
+                .should.eql('http://example.com/my/subdir/my/path.png', 'without root trailing-slash');
+
+            deduplicateSubdirectory(url, 'http://example.com/my/subdir/')
+                .should.eql('http://example.com/my/subdir/my/path.png', 'with root trailing-slash');
+        });
+
+        it('handles file that matches subdirectory', function () {
+            let url = 'http://example.com/my/path/my/path.png';
+
+            deduplicateSubdirectory(url, 'http://example.com/my/path')
+                .should.eql('http://example.com/my/path/my/path.png', 'without root trailing-slash');
+
+            deduplicateSubdirectory(url, 'http://example.com/my/path/')
+                .should.eql('http://example.com/my/path/my/path.png', 'with root trailing-slash');
+        });
+
+        it('handles subdirectory that matches tld', function () {
+            let url = 'http://example.blog/blog/file.png';
+
+            deduplicateSubdirectory(url, 'http://example.blog/blog/subdir')
+                .should.eql('http://example.blog/blog/file.png', 'without root trailing-slash');
+
+            deduplicateSubdirectory(url, 'http://example.blog/blog/subdir/')
+                .should.eql('http://example.blog/blog/file.png', 'with root trailing-slash');
+        });
+
+        it('keeps query and hash params', function () {
+            let url = 'http://example.blog/blog/blog/file.png?test=true#testing';
+
+            deduplicateSubdirectory(url, 'http://example.blog/blog/subdir')
+                .should.eql('http://example.blog/blog/blog/file.png?test=true#testing', 'without root trailing-slash');
+
+            deduplicateSubdirectory(url, 'http://example.blog/blog/subdir/')
+                .should.eql('http://example.blog/blog/blog/file.png?test=true#testing', 'with root trailing-slash');
+        });
+    });
+
+    describe('with path', function () {
+        it('ignores rootUrl with no subdirectory', function () {
+            let path = '/my/my/path.png';
+
+            deduplicateSubdirectory(path, 'https://example.com')
+                .should.eql('/my/my/path.png', 'without root trailing-slash');
+
+            deduplicateSubdirectory(path, 'https://example.com/')
+                .should.eql('/my/my/path.png', 'with root trailing-slash');
+        });
+
+        it('deduplicates single directory', function () {
+            let path = '/subdir/subdir/my/path.png';
+
+            deduplicateSubdirectory(path, 'https://example.com/subdir')
+                .should.eql('/subdir/my/path.png', 'without root trailing-slash');
+
+            deduplicateSubdirectory(path, 'https://example.com/subdir/')
+                .should.eql('/subdir/my/path.png', 'with root trailing-slash');
+        });
+
+        it('deduplicates multiple directories', function () {
+            let path = '/my/subdir/my/subdir/my/path.png';
+
+            deduplicateSubdirectory(path, 'http://example.com/my/subdir')
+                .should.eql('/my/subdir/my/path.png', 'without root trailing-slash');
+
+            deduplicateSubdirectory(path, 'http://example.com/my/subdir/')
+                .should.eql('/my/subdir/my/path.png', 'with root trailing-slash');
+        });
+
+        it('handles file that matches subdirectory', function () {
+            let path = '/my/path/my/path.png';
+
+            deduplicateSubdirectory(path, 'http://example.com/my/path')
+                .should.eql('/my/path/my/path.png', 'without root trailing-slash');
+
+            deduplicateSubdirectory(path, 'http://example.com/my/path/')
+                .should.eql('/my/path/my/path.png', 'with root trailing-slash');
+        });
+
+        it('handles subdirectory that matches tld', function () {
+            let path = '/blog/file.png';
+
+            deduplicateSubdirectory(path, 'http://example.blog/blog/subdir')
+                .should.eql('/blog/file.png', 'without root trailing-slash');
+
+            deduplicateSubdirectory(path, 'http://example.blog/blog/subdir/')
+                .should.eql('/blog/file.png', 'with root trailing-slash');
+        });
+
+        it('keeps query and hash params', function () {
+            let path = '/blog/blog/file.png?test=true#testing';
+
+            deduplicateSubdirectory(path, 'http://example.blog/blog/subdir')
+                .should.eql('/blog/blog/file.png?test=true#testing', 'without root trailing-slash');
+
+            deduplicateSubdirectory(path, 'http://example.blog/blog/subdir/')
+                .should.eql('/blog/blog/file.png?test=true#testing', 'with root trailing-slash');
+        });
+
+        it('deduplicates path with no trailing slash that matches subdir', function () {
+            deduplicateSubdirectory('/blog/blog', 'http://example.com/blog')
+                .should.equal('/blog/', 'without root trailing-slash');
+
+            deduplicateSubdirectory('/blog/blog', 'http://example.com/blog/')
+                .should.equal('/blog/', 'with root trailing-slash');
+        });
+    });
+});
diff --git a/ghost/config-url-hepers/lib/utils/deduplicate-subdirectory.js b/ghost/config-url-hepers/lib/utils/deduplicate-subdirectory.js
new file mode 100644
index 0000000000..d86ee4a744
--- /dev/null
+++ b/ghost/config-url-hepers/lib/utils/deduplicate-subdirectory.js
@@ -0,0 +1,31 @@
+const {URL} = require('url');
+
+/**
+ * Remove duplicated directories from the start of a path or url's path
+ *
+ * @param {string} url URL or pathname with possible duplicate subdirectory
+ * @param {string} rootUrl Root URL with an optional subdirectory
+ * @returns {string} URL or pathname with any duplicated subdirectory removed
+ */
+const deduplicateSubdirectory = function deduplicateSubdirectory(url, rootUrl) {
+    // force root url to always have a trailing-slash for consistent behaviour
+    if (!rootUrl.endsWith('/')) {
+        rootUrl = `${rootUrl}/`;
+    }
+
+    const parsedRoot = new URL(rootUrl);
+
+    // do nothing if rootUrl does not have a subdirectory
+    if (parsedRoot.pathname === '/') {
+        return url;
+    }
+
+    const subdir = parsedRoot.pathname.replace(/(^\/|\/$)+/g, '');
+    // we can have subdirs that match TLDs so we need to restrict matches to
+    // duplicates that start with a / or the beginning of the url
+    const subdirRegex = new RegExp(`(^|/)${subdir}/${subdir}(/|$)`);
+
+    return url.replace(subdirRegex, `$1${subdir}/`);
+};
+
+module.exports = deduplicateSubdirectory;