Allow `or` in case of `from` clause with same value

activerecord/lib/active_record/relation/from_clause.rb

@@ -18,6 +18,10 @@ def empty?
         value.nil?
       end
 
+      def ==(other)
+        other.is_a?(FromClause) && name == other.name && value == other.value
+      end
+
       def self.empty
         @empty ||= new(nil, nil).freeze
       end

activerecord/test/cases/relation/or_test.rb

@@ -147,6 +147,7 @@ def test_structurally_incompatible_values
         Post.group(:author_id).group(:author_id).or(Post.group(:author_id))
         Post.joins(:author).joins(:author).or(Post.joins(:author))
         Post.left_outer_joins(:author).left_outer_joins(:author).or(Post.left_outer_joins(:author))
+        Post.from("posts").or(Post.from("posts"))
       end
     end
   end