diff --git a/backend/atlas_portal/keycloak.py b/backend/atlas_portal/keycloak.py
index 394f81a..0df0b97 100644
--- a/backend/atlas_portal/keycloak.py
+++ b/backend/atlas_portal/keycloak.py
@@ -106,6 +106,30 @@ class KeycloakAdminClient:
             user = users[0]
             return user if isinstance(user, dict) else None
 
+    def find_user_by_email(self, email: str) -> dict[str, Any] | None:
+        email = (email or "").strip()
+        if not email:
+            return None
+
+        url = f"{settings.KEYCLOAK_ADMIN_URL}/admin/realms/{settings.KEYCLOAK_REALM}/users"
+        # Match the portal's username query behavior: set a low `max` and post-filter for exact matches.
+        params = {"email": email, "exact": "true", "max": "2"}
+        email_norm = email.lower()
+
+        with httpx.Client(timeout=settings.HTTP_CHECK_TIMEOUT_SEC) as client:
+            resp = client.get(url, params=params, headers=self._headers())
+            resp.raise_for_status()
+            users = resp.json()
+            if not isinstance(users, list) or not users:
+                return None
+            for user in users:
+                if not isinstance(user, dict):
+                    continue
+                candidate = user.get("email")
+                if isinstance(candidate, str) and candidate.strip().lower() == email_norm:
+                    return user
+        return None
+
     def get_user(self, user_id: str) -> dict[str, Any]:
         url = f"{settings.KEYCLOAK_ADMIN_URL}/admin/realms/{settings.KEYCLOAK_REALM}/users/{quote(user_id, safe='')}"
         with httpx.Client(timeout=settings.HTTP_CHECK_TIMEOUT_SEC) as client:
diff --git a/backend/atlas_portal/provisioning.py b/backend/atlas_portal/provisioning.py
index ead8b24..80af303 100644
--- a/backend/atlas_portal/provisioning.py
+++ b/backend/atlas_portal/provisioning.py
@@ -71,7 +71,25 @@ def _safe_error_detail(exc: Exception, fallback: str) -> str:
         if msg:
             return msg
     if isinstance(exc, httpx.HTTPStatusError):
-        return f"http {exc.response.status_code}"
+        detail = f"http {exc.response.status_code}"
+        try:
+            payload = exc.response.json()
+            msg: str | None = None
+            if isinstance(payload, dict):
+                raw = payload.get("errorMessage") or payload.get("error") or payload.get("message")
+                if isinstance(raw, str) and raw.strip():
+                    msg = raw.strip()
+            elif isinstance(payload, str) and payload.strip():
+                msg = payload.strip()
+            if msg:
+                msg = " ".join(msg.split())
+                detail = f"{detail}: {msg[:200]}"
+        except Exception:
+            text = (exc.response.text or "").strip()
+            if text:
+                text = " ".join(text.split())
+                detail = f"{detail}: {text[:200]}"
+        return detail
     if isinstance(exc, httpx.TimeoutException):
         return "timeout"
     return fallback
@@ -181,6 +199,9 @@ def provision_access_request(request_code: str) -> ProvisionResult:
                     email = contact_email.strip()
                     if not email:
                         raise RuntimeError("missing verified email address")
+                    existing_email_user = admin_client().find_user_by_email(email)
+                    if existing_email_user and (existing_email_user.get("username") or "") != username:
+                        raise RuntimeError("email is already associated with an existing Atlas account")
                     # Always enforce email verification in Keycloak itself (even if the portal
                     # already verified an external email before approval).
                     required_actions = ["UPDATE_PASSWORD", "VERIFY_EMAIL", "CONFIGURE_TOTP"]
diff --git a/backend/atlas_portal/routes/access_requests.py b/backend/atlas_portal/routes/access_requests.py
index 83c9ba9..015a4d0 100644
--- a/backend/atlas_portal/routes/access_requests.py
+++ b/backend/atlas_portal/routes/access_requests.py
@@ -236,6 +236,8 @@ def register(app) -> None:
 
         if admin_client().ready() and admin_client().find_user(username):
             return jsonify({"error": "username already exists"}), 409
+        if admin_client().ready() and admin_client().find_user_by_email(email):
+            return jsonify({"error": "email is already associated with an existing Atlas account"}), 409
 
         try:
             with connect() as conn: