diff --git a/lib/generators/unread/migration/templates/migration.rb b/lib/generators/unread/migration/templates/migration.rb
index ecdb1db..efe3413 100644
--- a/lib/generators/unread/migration/templates/migration.rb
+++ b/lib/generators/unread/migration/templates/migration.rb
@@ -6,7 +6,7 @@ def self.up
       t.datetime :timestamp
     end
 
-    add_index ReadMark, [:reader_id, :reader_type, :readable_type, :readable_id], name: 'read_marks_reader_readable_index'
+    add_index ReadMark, [:reader_id, :reader_type, :readable_type, :readable_id], name: 'read_marks_reader_readable_index', unique: true
   end
 
   def self.down
diff --git a/lib/unread/readable.rb b/lib/unread/readable.rb
index 66f651f..4a7ab64 100644
--- a/lib/unread/readable.rb
+++ b/lib/unread/readable.rb
@@ -27,11 +27,18 @@ def mark_array_as_read(array, reader)
             if global_timestamp && global_timestamp >= timestamp
               # The object is implicitly marked as read, so there is nothing to do
             else
-              rm = obj.read_marks.where(:reader_id => reader.id, :reader_type => reader.class.base_class.name).first || obj.read_marks.build
-              rm.reader_id   = reader.id
-              rm.reader_type = reader.class.base_class.name
-              rm.timestamp   = timestamp
-              rm.save!
+              # This transaction is needed, so that parent transaction won't rollback even there's an error.
+              ReadMark.transaction(requires_new: true) do
+                begin
+                  rm = obj.read_marks.where(reader_id: reader.id, reader_type: reader.class.base_class.name).first || obj.read_marks.build
+                  rm.reader_id   = reader.id
+                  rm.reader_type = reader.class.base_class.name
+                  rm.timestamp   = timestamp
+                  rm.save!
+                rescue ActiveRecord::RecordNotUnique
+                  raise ActiveRecord::Rollback
+                end
+              end
             end
           end
         end
diff --git a/spec/unread/readable_spec.rb b/spec/unread/readable_spec.rb
index a97416a..4fbc348 100644
--- a/spec/unread/readable_spec.rb
+++ b/spec/unread/readable_spec.rb
@@ -266,6 +266,20 @@
       expect(@email2.unread?(@reader)).to be_falsey
     end
 
+    it "should mark the rest as read when the first record is not unique" do
+      Email.mark_as_read! [ @email1 ], for: @reader
+
+      allow(@email1).to receive_message_chain("read_marks.build").and_return(@email1.read_marks.build)
+      allow(@email1).to receive_message_chain("read_marks.where").and_return([])
+
+      expect do
+        Email.mark_as_read! [ @email1, @email2 ], for: @reader
+      end.to change(ReadMark, :count).by(1)
+
+      expect(@email1.unread?(@reader)).to be_falsey
+      expect(@email2.unread?(@reader)).to be_falsey
+    end
+
     it "should perform less queries if the objects are already read" do
       Email.mark_as_read! :all, :for => @reader