feat(gen-deps-list): detect duplicate packages

Prevents a bug where a second entry would override the previous one and possibly
decrease the release type (e.g. `major + patch → patch`).

scripts/gen-deps-list.js

@@ -123,6 +123,10 @@ async function readPackagesFromChangelog(toRelease) {
     }
 
     const { name, releaseType } = match.groups
+    if (name in toRelease) {
+      throw new Error('duplicate package to release in CHANGELOG.unreleased.md: ' + name)
+    }
+
     toRelease[name] = releaseType
   })
 }