diff --git a/activerecord/lib/active_record/relation/spawn_methods.rb b/activerecord/lib/active_record/relation/spawn_methods.rb
index b8b0898cc8..648a02f1cc 100644
--- a/activerecord/lib/active_record/relation/spawn_methods.rb
+++ b/activerecord/lib/active_record/relation/spawn_methods.rb
@@ -26,21 +26,19 @@ module ActiveRecord
 
       merged_relation = merged_relation.joins(r.joins_values)
 
-      merged_wheres = @where_values.dup + r.where_values
+      merged_wheres = @where_values + r.where_values
 
-      equality_wheres = merged_wheres.find_all { |w|
-        w.respond_to?(:operator) && w.operator == :==
-      }
-
-      equality_wheres_by_operand = equality_wheres.group_by { |eq|
-        eq.left.name
-      }
-
-      duplicates = equality_wheres_by_operand.map { |name, list|
-        list[0...-1]
-      }.flatten
-
-      merged_wheres -= duplicates
+      # Remove duplicates, last one wins.
+      seen = {}
+      merged_wheres = merged_wheres.reverse.reject { |w|
+        nuke = false
+        if w.respond_to?(:operator) && w.operator == :==
+          name       = w.left.name
+          nuke       = seen[name]
+          seen[name] = true
+        end
+        nuke
+      }.reverse
 
       merged_relation.where_values = merged_wheres