1 files changed, 27 insertions, 0 deletions
diff --git a/danger/duplicate_yarn_dependencies/Dangerfile b/danger/duplicate_yarn_dependencies/Dangerfile
new file mode 100644
index 00000000000..25f81ec86a4
--- /dev/null
+++ b/danger/duplicate_yarn_dependencies/Dangerfile
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+return unless helper.all_changed_files.include? 'yarn.lock'
+
+duplicate = `node_modules/.bin/yarn-deduplicate --list --strategy fewer yarn.lock`
+             .split(/$/)
+             .map(&:strip)
+             .reject(&:empty?)
+
+return if duplicate.empty?
+
+warn 'This merge request has introduced duplicated yarn dependencies.'
+
+markdown(<<~MARKDOWN)
+  ## Duplicate yarn dependencies
+
+  The following dependencies should be de-duplicated:
+
+  * #{duplicate.map { |path| "`#{path}`" }.join("\n* ")}
+
+  Please run the following command and commit the changes to `yarn.lock`:
+
+  ```
+  node_modules/.bin/yarn-deduplicate --strategy fewer yarn.lock \\
+    && yarn install
+  ```
+MARKDOWN