diff --git a/backend/src/sandbox.ts b/backend/src/sandbox.ts
index f147a80..a2774f4 100644
--- a/backend/src/sandbox.ts
+++ b/backend/src/sandbox.ts
@@ -4,7 +4,7 @@ import * as fs from "fs";
 import { v4 as getUUID } from "uuid";
 
 import { langs } from "./langs";
-import { borrowUser } from "./users";
+import { MIN_UID, MAX_UID, borrowUser, ignoreUsers } from "./users";
 import {
   getEnv,
   privilegedSetup,
@@ -23,6 +23,22 @@ function log(msg: any) {
 }
 
 async function main() {
+  const dirs = await new Promise<string[]>((resolve, reject) =>
+    fs.readdir("/tmp/riju", (err, dirs) => (err ? reject(err) : resolve(dirs)))
+  );
+  const uids = (
+    await Promise.all(
+      dirs.map(
+        (dir) =>
+          new Promise<number>((resolve, reject) =>
+            fs.stat(`/tmp/riju/${dir}`, (err, stat) =>
+              err ? reject(err) : resolve(stat.uid)
+            )
+          )
+      )
+    )
+  ).filter((uid) => uid >= MIN_UID && uid < MAX_UID);
+  await ignoreUsers(uids, log);
   const uuid = getUUID();
   const { uid, returnUID } = await borrowUser(log);
   await run(privilegedSetup({ uid, uuid }), log);
diff --git a/backend/src/users.ts b/backend/src/users.ts
index d41d07d..aecbc5a 100644
--- a/backend/src/users.ts
+++ b/backend/src/users.ts
@@ -10,8 +10,8 @@ import { PRIVILEGED } from "./config";
 import { privilegedUseradd, run } from "./util";
 
 // Keep in sync with system/src/riju-system-privileged.c
-const MIN_UID = 2000;
-const MAX_UID = 65000;
+export const MIN_UID = 2000;
+export const MAX_UID = 65000;
 
 const CUR_UID = os.userInfo().uid;
 
@@ -33,9 +33,10 @@ async function readExistingUsers(log: (msg: string) => void) {
   )
     .filter(({ username }) => username.startsWith("riju"))
     .map(({ uid }) => parseInt(uid))
-    .filter((uid) => !isNaN(uid) && uid >= MIN_UID && uid < MAX_UID);
+    .filter((uid) => !isNaN(uid) && uid >= MIN_UID && uid < MAX_UID)
+    .reverse();
   nextId = (_.max(availIds) || MIN_UID - 1) + 1;
-  log(`Found ${availIds.length} existing users, next ID is ${nextId}`);
+  log(`Found ${availIds.length} existing users, next is riju${nextId}`);
 }
 
 async function createUser(log: (msg: string) => void): Promise<number> {
@@ -49,6 +50,25 @@ async function createUser(log: (msg: string) => void): Promise<number> {
   return uid;
 }
 
+export async function ignoreUsers(uids: number[], log: (msg: string) => void) {
+  await lock.acquire("key", async () => {
+    if (availIds === null || nextId === null) {
+      await readExistingUsers(log);
+    }
+    const uidSet = new Set(uids);
+    if (uidSet.size > 0) {
+      const plural = uidSet.size !== 1 ? "s" : "";
+      log(
+        `Ignoring user${plural} from open session${plural}: ${Array.from(uidSet)
+          .sort()
+          .map((uid) => `riju${uid}`)
+          .join(", ")}`
+      );
+    }
+    availIds = availIds!.filter((uid) => !uidSet.has(uid));
+  });
+}
+
 export async function borrowUser(log: (msg: string) => void) {
   if (!PRIVILEGED) {
     return { uid: CUR_UID, returnUID: async () => {} };