diff --git a/activerecord/lib/active_record/relation/spawn_methods.rb b/activerecord/lib/active_record/relation/spawn_methods.rb
index ba882beca9285..5d94bbed37deb 100644
--- a/activerecord/lib/active_record/relation/spawn_methods.rb
+++ b/activerecord/lib/active_record/relation/spawn_methods.rb
@@ -31,22 +31,7 @@ def merge(r)
 
       merged_wheres = @where_values + r.where_values
 
-      unless @where_values.empty?
-        # Remove duplicates, last one wins.
-        seen = Hash.new { |h,table| h[table] = {} }
-        merged_wheres = merged_wheres.reverse.reject { |w|
-          nuke = false
-          if w.respond_to?(:operator) && w.operator == :==
-            name              = w.left.name
-            table             = w.left.relation.name
-            nuke              = seen[table][name]
-            seen[table][name] = true
-          end
-          nuke
-        }.reverse
-      end
-
-      merged_relation.where_values = merged_wheres
+      merged_relation.where_values = merged_wheres.uniq
 
       (Relation::SINGLE_VALUE_METHODS - [:lock, :create_with]).each do |method|
         value = r.send(:"#{method}_value")
diff --git a/activerecord/test/cases/associations/eager_test.rb b/activerecord/test/cases/associations/eager_test.rb
index 289ee359a5abe..6066effd86846 100644
--- a/activerecord/test/cases/associations/eager_test.rb
+++ b/activerecord/test/cases/associations/eager_test.rb
@@ -981,10 +981,16 @@ def test_preload_has_many_with_association_condition_and_default_scope
     LazyReader.create! :person => people(:susan), :post => post
 
     assert_equal 1, post.lazy_readers.to_a.size
-    assert_equal 2, post.lazy_readers_skimmers_or_not.to_a.size
+
+    # This test relied on some completely broken and incorrect behaviour in
+    # ActiveRecord that we've fixed.
+    #
+    # assert_equal 2, post.lazy_readers_skimmers_or_not.to_a.size
+
+    assert_equal 1, post.lazy_readers_skimmers_or_not.to_a.size
 
     post_with_readers = Post.includes(:lazy_readers_skimmers_or_not).find(post.id)
-    assert_equal 2, post_with_readers.lazy_readers_skimmers_or_not.to_a.size
+    assert_equal 1, post_with_readers.lazy_readers_skimmers_or_not.to_a.size
   end
 
   def test_include_has_many_using_primary_key
diff --git a/activerecord/test/cases/named_scope_test.rb b/activerecord/test/cases/named_scope_test.rb
index 8a6288bfff7d3..cce3c488d9762 100644
--- a/activerecord/test/cases/named_scope_test.rb
+++ b/activerecord/test/cases/named_scope_test.rb
@@ -364,11 +364,11 @@ def test_chaining_should_use_latest_conditions_when_creating
 
   def test_chaining_should_use_latest_conditions_when_searching
     # Normal hash conditions
-    assert_equal Topic.where(:approved => true).to_a, Topic.rejected.approved.all
-    assert_equal Topic.where(:approved => false).to_a, Topic.approved.rejected.all
+    assert_equal [], Topic.rejected.approved.all
+    assert_equal [], Topic.approved.rejected.all
 
     # Nested hash conditions with same keys
-    assert_equal [posts(:sti_comments)], Post.with_special_comments.with_very_special_comments.all
+    assert_equal [], Post.with_special_comments.with_very_special_comments.all
 
     # Nested hash conditions with different keys
     assert_equal [posts(:sti_comments)], Post.with_special_comments.with_post(4).all.uniq
diff --git a/activerecord/test/cases/relation_scoping_test.rb b/activerecord/test/cases/relation_scoping_test.rb
index 1e2093273ed48..698984956fb7d 100644
--- a/activerecord/test/cases/relation_scoping_test.rb
+++ b/activerecord/test/cases/relation_scoping_test.rb
@@ -278,7 +278,13 @@ def test_should_maintain_default_scope_on_associations
 
   def test_should_default_scope_on_associations_is_overriden_by_association_conditions
     reference = references(:michael_unicyclist).becomes(BadReference)
-    assert_equal [reference], people(:michael).fixed_bad_references
+
+    # This test relied on some completely broken and incorrect behaviour in
+    # ActiveRecord that we've fixed.
+    #
+    # assert_equal [reference], people(:michael).fixed_bad_references
+
+    assert_equal [], people(:michael).fixed_bad_references
   end
 
   def test_should_maintain_default_scope_on_eager_loaded_associations