diff --git a/requirements.txt b/requirements.txt
index 5d9efb5f..f2821b20 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,5 @@
 fastapi
 uvicorn
 httpx
-watchfiles
\ No newline at end of file
+watchfiles
+pytest
\ No newline at end of file
diff --git a/src/app.py b/src/app.py
index 4ebb1d9b..f0fa0ac6 100644
--- a/src/app.py
+++ b/src/app.py
@@ -62,6 +62,13 @@ def signup_for_activity(activity_name: str, email: str):
     # Get the specific activity
     activity = activities[activity_name]
 
+    normalized_email = email.strip().lower()
+    existing_participants = {participant.strip().lower() for participant in activity["participants"]}
+
+    # Validate student is not already signed up
+    if normalized_email in existing_participants:
+        raise HTTPException(status_code=400, detail="Student is already signed up")
+
     # Add student
     activity["participants"].append(email)
     return {"message": f"Signed up {email} for {activity_name}"}
diff --git a/tests/test_app.py b/tests/test_app.py
new file mode 100644
index 00000000..f51e7e57
--- /dev/null
+++ b/tests/test_app.py
@@ -0,0 +1,20 @@
+from uuid import uuid4
+
+from fastapi.testclient import TestClient
+
+from src.app import app
+
+client = TestClient(app)
+
+
+def test_student_cannot_sign_up_twice_for_same_activity():
+    activity_name = "Chess Club"
+    email = f"student-{uuid4()}@mergington.edu"
+
+    first_response = client.post(f"/activities/{activity_name}/signup?email={email}")
+    assert first_response.status_code == 200
+
+    second_response = client.post(f"/activities/{activity_name}/signup?email={email}")
+
+    assert second_response.status_code == 400
+    assert "already signed up" in second_response.json()["detail"].lower()