Add duplicate root route declarations pattern

Rails 4 raises ArgumentError: Invalid route name, already in use:
'root' when the routes file has more than one `root` declaration
(typically one per constraint block). Rails 3.2 was lenient about
duplicate names with different constraints, so this slips through on
many 3.x codebases.

The naive fix — give each duplicate an `as:` — silently breaks every
root_path / root_url caller in the codebase because the :root name
is no longer registered to any declaration. Real apps have dozens to
hundreds of these helper calls in controllers, views, mailers, and
tests.

Correct fix: keep the FIRST declaration as the default :root, only
add `as:` to the SECOND and subsequent. Includes a grep verification
step so operators confirm no caller targets a constraint-bound root
that was renamed.

Classified high_priority / kind: breaking — Rails 4 boot raises
outright on the duplicate.

Author: JuanVqz

rails-upgrade/detection-scripts/patterns/rails-40-patterns.yml

@@ -172,6 +172,16 @@ upgrade_findings:
       fix: "Add via: :get (or appropriate method) or replace with get/post/put/patch/delete helper"
       variable_name: "MATCH_WITHOUT_VIA"
 
+    - name: "Duplicate root route declarations across constraints"
+      kind: "breaking"
+      pattern: "^\\s*root\\s+(to:|=>)"
+      exclude: ""
+      search_paths:
+        - "config/"
+      explanation: "Rails 4 raises `ArgumentError: Invalid route name, already in use: 'root'` when the routes file has more than one `root` declaration (typically one per constraint block). Rails 3.2 was lenient about duplicate names with different constraints"
+      fix: "Keep the FIRST `root` declaration as the default `:root` name; add `as: :<unique_name>` to the SECOND and subsequent declarations. Renaming all of them (giving the first an `as:` too) silently breaks every `root_path` / `root_url` call site because the `:root` name is no longer registered. Real apps have dozens to hundreds of these helper calls. Verification step: `grep -rnE \"\\b(root_path|root_url)\\b\" app/ config/ lib/` and confirm no caller targets a constraint-bound root that was renamed"
+      variable_name: "DUPLICATE_ROOT_ROUTES"
+
   medium_priority:
     - name: "rescue_action method"
       kind: "breaking"