diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index 71aaedee1e..7531f22494 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/activerecord/lib/active_record/relation/merger.rb
@@ -97,11 +97,14 @@ module ActiveRecord
           merged_wheres = relation.where_values + values[:where]
 
           unless relation.where_values.empty?
-            # Remove duplicates, last one wins.
+            # Remove duplicate ARel attributes. 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 == :==
+              # We might have non-attributes on the left side of equality nodes,
+              # so we need to make sure they quack like an attribute.
+              if w.respond_to?(:operator) && w.operator == :== &&
+                w.left.respond_to?(:relation)
                 name              = w.left.name
                 table             = w.left.relation.name
                 nuke              = seen[table][name]
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 84027ea5ae..4ef11590a9 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -668,6 +668,22 @@ class RelationTest < ActiveRecord::TestCase
     assert_equal [developers(:poor_jamis)], dev_with_count.to_a
   end
 
+  def test_relation_merging_with_arel_equalities_keeps_last_equality
+    devs = Developer.where(Developer.arel_table[:salary].eq(80000)).merge(
+      Developer.where(Developer.arel_table[:salary].eq(9000))
+    )
+    assert_equal [developers(:poor_jamis)], devs.to_a
+  end
+
+  def test_relation_merging_with_arel_equalities_with_a_non_attribute_left_hand_ignores_non_attributes_when_discarding_equalities
+    salary_attr = Developer.arel_table[:salary]
+    devs = Developer.where(salary_attr.eq(80000)).merge(
+      Developer.where(salary_attr.eq(9000)).
+        where(Arel::Nodes::NamedFunction.new('abs', [salary_attr]).eq(9000))
+    )
+    assert_equal [developers(:poor_jamis)], devs.to_a
+  end
+
   def test_relation_merging_with_eager_load
     relations = []
     relations << Post.order('comments.id DESC').merge(Post.eager_load(:last_comment)).merge(Post.all)