diff --git a/tools/config.py b/tools/config.py
index dbc5c0afef..905bc20b6e 100644
--- a/tools/config.py
+++ b/tools/config.py
@@ -444,7 +444,7 @@ class Config:
             self.add_config_files(resources.json_files)
 
             # Add features while we find new ones
-            features = self.get_features()
+            features = set(self.get_features())
             if features == prev_features:
                 break
 
diff --git a/tools/test/config_test/test28/mbed_app.json b/tools/test/config_test/test28/mbed_app.json
new file mode 100644
index 0000000000..4e2addae59
--- /dev/null
+++ b/tools/test/config_test/test28/mbed_app.json
@@ -0,0 +1,17 @@
+{
+    "custom_targets": {
+        "test_target": {
+            "core": "Cortex-M0",
+            "extra_labels": [],
+            "features": [],
+            "default_build": "standard"
+        }
+    },
+    "target_overrides": {
+        "*": {
+            "target.features_add": ["UVISOR"],
+            "target.extra_labels_add": ["UVISOR_SUPPORTED"]
+        }
+    }
+}
+
diff --git a/tools/test/config_test/test28/test_data.py b/tools/test/config_test/test28/test_data.py
new file mode 100644
index 0000000000..4f69bfceef
--- /dev/null
+++ b/tools/test/config_test/test28/test_data.py
@@ -0,0 +1,8 @@
+# Testing when adding two features
+
+expected_results = {
+    "test_target": {
+        "desc": "test uvisor feature",
+        "expected_features": ["UVISOR"]
+    }
+}