diff --git a/CHANGELOG.md b/CHANGELOG.md
index 15a5caa..3fed75b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -26,9 +26,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `ensure_parent_dir` helper that no-ops on an empty parent.
 - Open5GS/free5gc adapters now use the configured `profile.apn` for the core
   session/DNN name instead of a hardcoded `"internet"`.
+- Connections set `PRAGMA busy_timeout` (5s) so concurrent writers under a
+  threaded server wait for the lock instead of raising "database is locked".
+- Provisioning retries on the (astronomically rare) IMSI/ICCID collision
+  instead of returning a 500.
+- The in-memory profile cache is guarded by a lock (thread-safe under a
+  threaded server), and the captive portal disables the activate button while
+  a request is in flight to prevent double provisioning.
 - Added `test_app.py` (API-level: provision, rate-limit, token download,
-  404 on unknown/enumerated ids), `test_utils.py` (MAC/IMSI/op_key/config/
-  ensure_parent_dir), and adapter APN coverage.
+  404 on unknown/enumerated ids, collision retry/give-up), `test_utils.py`
+  (MAC/IMSI/op_key/config/ensure_parent_dir), and adapter APN coverage.
 
 ### Added
 - **Phase-2 profile generation.** `create_esim_profile` now emits a structured
diff --git a/portal/js/script.js b/portal/js/script.js
index e71488d..3f639aa 100644
--- a/portal/js/script.js
+++ b/portal/js/script.js
@@ -15,7 +15,13 @@ function showError(msg) {
   showState('error');
 }
 
+var inFlight = false;
+
 async function activate() {
+  if (inFlight) return;  // ignore double-clicks while a request is pending
+  inFlight = true;
+  var btn = document.getElementById('btn-activate');
+  if (btn) btn.disabled = true;
   showState('loading');
 
   try {
@@ -49,6 +55,9 @@ async function activate() {
 
   } catch (err) {
     showError('Could not reach the server. Make sure you are connected to the WiFi network.');
+  } finally {
+    inFlight = false;
+    if (btn) btn.disabled = false;
   }
 }
 
diff --git a/server/app.py b/server/app.py
index ff953c0..dd0282a 100644
--- a/server/app.py
+++ b/server/app.py
@@ -2,6 +2,7 @@ import io
 import logging
 import os
 import secrets
+import threading
 import time
 from collections import OrderedDict
 from datetime import datetime, timezone
@@ -43,14 +44,21 @@ CONFIG_PATH = os.environ.get("CONFIG_PATH", "config.yaml")
 # The cache is size-capped so it cannot grow without bound.
 _MAX_CACHED_PROFILES = 256
 _profile_cache: "OrderedDict[str, bytes]" = OrderedDict()
+_cache_lock = threading.Lock()
 _start_time = time.time()
 
 
 def _cache_profile(token: str, data: bytes) -> None:
-    _profile_cache[token] = data
-    _profile_cache.move_to_end(token)
-    while len(_profile_cache) > _MAX_CACHED_PROFILES:
-        _profile_cache.popitem(last=False)
+    with _cache_lock:
+        _profile_cache[token] = data
+        _profile_cache.move_to_end(token)
+        while len(_profile_cache) > _MAX_CACHED_PROFILES:
+            _profile_cache.popitem(last=False)
+
+
+def _get_cached_profile(token: str):
+    with _cache_lock:
+        return _profile_cache.get(token)
 
 
 def create_app(config_path: str = CONFIG_PATH) -> Flask:
@@ -120,20 +128,29 @@ def create_app(config_path: str = CONFIG_PATH) -> Flask:
                     {"error": "Rate limit exceeded. One eSIM profile per device per hour."}
                 ), 429
 
-            imsi = generate_imsi(mcc, mnc)
-            ki = generate_ki()
-            iccid = generate_iccid()
-            opc = derive_opc(ki, op_bytes)
-
-            ki_hex = bytes_to_hex(ki)
-            opc_hex = bytes_to_hex(opc)
+            # Retry on the astronomically-unlikely IMSI/ICCID collision so a
+            # duplicate does not surface to the user as a 500.
+            sub_id = None
+            for attempt in range(5):
+                imsi = generate_imsi(mcc, mnc)
+                ki = generate_ki()
+                iccid = generate_iccid()
+                opc = derive_opc(ki, op_bytes)
+                ki_hex = bytes_to_hex(ki)
+                opc_hex = bytes_to_hex(opc)
+                try:
+                    sub_id = add_subscriber(conn, imsi, iccid, ki_hex, opc_hex, mac)
+                    break
+                except ValueError as exc:
+                    logger.warning(f"Credential collision (attempt {attempt + 1}), retrying: {exc}")
+            if sub_id is None:
+                logger.error("Provision failed: could not allocate a unique IMSI/ICCID")
+                return jsonify({"error": "Profile generation failed"}), 500
 
             profile_bytes = create_esim_profile(
                 imsi, ki, opc, iccid, profile_name, carrier_name,
                 smdp_address=smdp_address, apn=apn,
             )
-
-            sub_id = add_subscriber(conn, imsi, iccid, ki_hex, opc_hex, mac)
             token = secrets.token_urlsafe(24)
             _cache_profile(token, profile_bytes)
 
@@ -164,7 +181,7 @@ def create_app(config_path: str = CONFIG_PATH) -> Flask:
         # Capability URL: only the holder of the random token can download the
         # profile. Never distinguish "wrong token" from "expired/evicted" —
         # both are a plain 404 so tokens can't be probed.
-        profile_bytes = _profile_cache.get(token)
+        profile_bytes = _get_cached_profile(token)
         if profile_bytes is None:
             return jsonify({"error": "Profile not found"}), 404
 
diff --git a/server/database.py b/server/database.py
index b2455be..c8528d6 100644
--- a/server/database.py
+++ b/server/database.py
@@ -35,9 +35,14 @@ def init_db(db_path: str) -> None:
 
 
 def get_connection(db_path: str) -> sqlite3.Connection:
-    """Return an open connection with Row factory enabled."""
-    conn = sqlite3.connect(db_path, detect_types=sqlite3.PARSE_DECLTYPES)
+    """Return an open connection with Row factory enabled.
+
+    Sets a busy timeout so that under a threaded server concurrent writers
+    wait for the lock instead of immediately raising "database is locked".
+    """
+    conn = sqlite3.connect(db_path, detect_types=sqlite3.PARSE_DECLTYPES, timeout=5.0)
     conn.row_factory = sqlite3.Row
+    conn.execute("PRAGMA busy_timeout = 5000")
     return conn
 
 
diff --git a/tests/test_app.py b/tests/test_app.py
index 5d0fba0..6c31692 100644
--- a/tests/test_app.py
+++ b/tests/test_app.py
@@ -71,6 +71,32 @@ class TestProvision:
         r = client.post("/api/provision", json={"mac_address": "de:ad:be:ef:00:01"})
         assert r.status_code == 429
 
+    def test_provision_retries_on_collision(self, client, monkeypatch):
+        import server.app as appmod
+        real_add = appmod.add_subscriber
+        calls = {"n": 0}
+
+        def flaky(conn, imsi, iccid, ki, opc, mac):
+            calls["n"] += 1
+            if calls["n"] == 1:
+                raise ValueError("Duplicate subscriber")
+            return real_add(conn, imsi, iccid, ki, opc, mac)
+
+        monkeypatch.setattr(appmod, "add_subscriber", flaky)
+        r = client.post("/api/provision", json={"mac_address": "aa:bb:cc:ab:cd:ef"})
+        assert r.status_code == 200
+        assert calls["n"] == 2  # failed once, succeeded on retry
+
+    def test_provision_gives_up_after_persistent_collision(self, client, monkeypatch):
+        import server.app as appmod
+
+        def always_dup(*a, **k):
+            raise ValueError("Duplicate subscriber")
+
+        monkeypatch.setattr(appmod, "add_subscriber", always_dup)
+        r = client.post("/api/provision", json={"mac_address": "aa:bb:cc:ab:cd:11"})
+        assert r.status_code == 500
+
 
 class TestProfileDownload:
     def test_download_with_token(self, client):