diff --git a/lib/blog/accounts/accounts.ex b/lib/blog/accounts/accounts.ex
index 69e5c52..5b4b529 100644
--- a/lib/blog/accounts/accounts.ex
+++ b/lib/blog/accounts/accounts.ex
@@ -13,6 +13,9 @@ defmodule Blog.Accounts do
       with {:ok, contact} <- create_contact(contact_attrs),
            {:ok, user} <- do_create_user(user_attrs, contact) do
         %{user | contacts: [contact]}
+      else
+        {:error, changeset} ->
+          Repo.rollback(changeset)
       end
     end
 
@@ -21,14 +24,14 @@ defmodule Blog.Accounts do
   def create_contact(attrs) do
     attrs
     |> Accounts.Contact.changeset
-    |> Blog.Repo.insert
+    |> Repo.insert
   end
 
   defp do_create_user(attrs, contact) do
     attrs
     |> Map.put(:contact_id, contact.id)
     |> Accounts.User.changeset
-    |> Blog.Repo.insert
+    |> Repo.insert
   end
 
 end
diff --git a/lib/blog/accounts/contact.ex b/lib/blog/accounts/contact.ex
index 9779055..c598b9f 100644
--- a/lib/blog/accounts/contact.ex
+++ b/lib/blog/accounts/contact.ex
@@ -24,6 +24,7 @@ defmodule Blog.Accounts.Contact do
     contact
     |> cast(attrs, [:type, :value])
     |> validate_required([:type, :value])
+    |> unique_constraint(:value, name: :contacts_type_value_index)
   end
 
 end
diff --git a/test/blog/accounts/accounts_test.exs b/test/blog/accounts/accounts_test.exs
new file mode 100644
index 0000000..b677efb
--- /dev/null
+++ b/test/blog/accounts/accounts_test.exs
@@ -0,0 +1,27 @@
+defmodule Blog.AccountsTest do
+  use Blog.DataCase
+
+  alias Blog.Accounts
+
+  test "create_user/1 returns {:error, changeset} and rolles back transaction on error" do
+    attrs = %{
+      contact: %{type: "phone", value: "+1 000 000 0000"},
+      name: "",
+      password: "password"
+    }
+
+    contact_count = Repo.aggregate(Accounts.Contact, :count, :id)
+
+    assert {:error, %Ecto.Changeset{}} = Accounts.create_user(attrs)
+    assert ^contact_count = Repo.aggregate(Accounts.Contact, :count, :id)
+  end
+
+  test "create_user/1 discards duplicate contacts" do
+    contact = %{type: "phone", value: "+1 000 000 0000"}
+
+    assert {:ok, _} = Accounts.create_user(%{contact: contact, name: "name", password: "password"})
+
+    assert {:error, changeset} = Accounts.create_user(%{contact: contact, name: "other", password: "password"})
+    assert "has already been taken" in errors_on(changeset).value
+  end
+end