diff --git a/lib/phoenix_ecto/sql/sandbox.ex b/lib/phoenix_ecto/sql/sandbox.ex
index 2c1bc34..928bda3 100644
--- a/lib/phoenix_ecto/sql/sandbox.ex
+++ b/lib/phoenix_ecto/sql/sandbox.ex
@@ -339,7 +339,7 @@ defmodule Phoenix.Ecto.SQL.Sandbox do
   end
 
   def allow(%{repo: repo, owner: owner}, sandbox),
-    do: Enum.each(List.wrap(repo), &sandbox.allow(&1, owner, self()))
+    do: Enum.each(List.wrap(repo), &sandbox.allow(&1, owner, self(), unallow_existing: true))
 
   def allow(%{}, _sandbox), do: :ok
   def allow(nil, _sandbox), do: :ok
diff --git a/test/phoenix_ecto/sql/sandbox_test.exs b/test/phoenix_ecto/sql/sandbox_test.exs
index 3e23474..192b0a4 100644
--- a/test/phoenix_ecto/sql/sandbox_test.exs
+++ b/test/phoenix_ecto/sql/sandbox_test.exs
@@ -15,7 +15,7 @@ defmodule PhoenixEcto.SQL.SandboxTest do
       :ok
     end
 
-    def allow(repo, owner, _allowed) do
+    def allow(repo, owner, _allowed, _opts \\ []) do
       send(owner, {:allowed, repo})
     end
   end