diff --git a/tests/test_installer.py b/tests/test_installer.py
new file mode 100644
index 0000000..59e8563
--- /dev/null
+++ b/tests/test_installer.py
@@ -0,0 +1,59 @@
+# pylint: disable=missing-module-docstring
+import time
+
+import tox.venv
+from poetry.factory import Factory
+
+from .fixtures import mock_poetry_factory
+from .fixtures import mock_venv
+from tox_poetry_installer import datatypes
+from tox_poetry_installer import installer
+
+
+def test_deduplication(mock_venv, mock_poetry_factory):
+    """Test that the installer does not install duplicate dependencies"""
+    poetry = Factory().create_poetry(None)
+    packages: datatypes.PackageMap = {
+        item.name: item for item in poetry.locker.locked_repository(False).packages
+    }
+
+    venv = tox.venv.VirtualEnv()
+    to_install = [packages["toml"], packages["toml"]]
+
+    installer.install(poetry, venv, to_install)
+
+    assert len(set(to_install)) == len(venv.installed)
+
+
+def test_parallelization(mock_venv, mock_poetry_factory):
+    """Test that behavior is consistent between parallel and non-parallel usage"""
+    poetry = Factory().create_poetry(None)
+    packages: datatypes.PackageMap = {
+        item.name: item for item in poetry.locker.locked_repository(False).packages
+    }
+
+    to_install = [
+        packages["toml"],
+        packages["toml"],
+        packages["tox"],
+        packages["requests"],
+        packages["python-dateutil"],
+        packages["attrs"],
+    ]
+
+    venv_sequential = tox.venv.VirtualEnv()
+    start_sequential = time.time()
+    installer.install(poetry, venv_sequential, to_install, None)
+    sequential = time.time() - start_sequential
+
+    venv_parallel = tox.venv.VirtualEnv()
+    start_parallel = time.time()
+    installer.install(poetry, venv_parallel, to_install, 5)
+    parallel = time.time() - start_parallel
+
+    # The mock delay during package install is static (one second) so these values should all
+    # be within microseconds of each other
+    assert parallel < sequential
+    assert round(parallel * 5) == round(sequential)
+    assert round(sequential) == len(set(to_install))
+    assert round(parallel * 5) == len(set(to_install))