diff --git a/src/services/NotificationService.ts b/src/services/NotificationService.ts
index 1f52eef..6fee796 100644
--- a/src/services/NotificationService.ts
+++ b/src/services/NotificationService.ts
@@ -3,7 +3,7 @@ import jwt from "jsonwebtoken";
 import fs from "fs";
 import { TupleKey } from "../types/TupleKey";
 import { IEta } from "../entities/entities";
-import * as http2 from "node:http2";
+import http2 from "http2";
 
 export interface ScheduledNotificationData {
   deviceId: string;
@@ -43,7 +43,7 @@ export class NotificationService {
    * stop ID, which can be generated using `TupleKey`.
    * @private
    */
-  private deviceIdsToDeliverTo: { [key: string]: string[] } = {}
+  private deviceIdsToDeliverTo: { [key: string]: Set<string> } = {}
 
   public reloadAPNsTokenIfTimePassed() {
     if (this.lastReloadedTimeForAPNsIsTooRecent()) {
@@ -176,15 +176,17 @@ export class NotificationService {
       return;
     }
 
-    const indicesToRemove = new Set();
-    await Promise.all(this.deviceIdsToDeliverTo[tuple.toString()].map(async (deviceId, index) => {
+    const deviceIdsToRemove = new Set<string>();
+    for (let deviceId of this.deviceIdsToDeliverTo[tuple.toString()].values()) {
       const deliveredSuccessfully = await this.sendEtaNotificationImmediatelyIfSecondsRemainingBelowThreshold(deviceId, eta);
       if (deliveredSuccessfully) {
-        indicesToRemove.add(index);
+        deviceIdsToRemove.add(deviceId);
       }
-    }));
+    }
 
-    this.deviceIdsToDeliverTo[tuple.toString()] = this.deviceIdsToDeliverTo[tuple.toString()].filter((_, index) => !indicesToRemove.has(index));
+    deviceIdsToRemove.forEach((deviceId) => {
+      this.deviceIdsToDeliverTo[tuple.toString()].delete(deviceId);
+    });
   }
 
   private async sendEtaNotificationImmediatelyIfSecondsRemainingBelowThreshold(deviceId: string, eta: IEta) {
@@ -208,10 +210,9 @@ export class NotificationService {
   public async scheduleNotification({ deviceId, shuttleId, stopId }: ScheduledNotificationData) {
     const tuple = new TupleKey(shuttleId, stopId);
     if (this.deviceIdsToDeliverTo[tuple.toString()] === undefined) {
-      this.deviceIdsToDeliverTo[tuple.toString()] = [deviceId];
-    } else {
-      this.deviceIdsToDeliverTo[tuple.toString()].push(deviceId);
+      this.deviceIdsToDeliverTo[tuple.toString()] = new Set();
     }
+    this.deviceIdsToDeliverTo[tuple.toString()].add(deviceId);
 
     this.repository.unsubscribeFromEtaUpdates(this.etaSubscriberCallback);
     this.repository.subscribeToEtaUpdates(this.etaSubscriberCallback);
@@ -227,15 +228,12 @@ export class NotificationService {
     const tupleKey = new TupleKey(shuttleId, stopId);
     if (
       this.deviceIdsToDeliverTo[tupleKey.toString()] === undefined
-      || !this.deviceIdsToDeliverTo[tupleKey.toString()].includes(deviceId)
+      || !this.deviceIdsToDeliverTo[tupleKey.toString()].has(deviceId)
     ) {
       return;
     }
 
-    const index = this.deviceIdsToDeliverTo[tupleKey.toString()].findIndex(id => id === deviceId);
-    if (index !== -1) {
-      this.deviceIdsToDeliverTo[tupleKey.toString()].splice(index, 1);
-    }
+    this.deviceIdsToDeliverTo[tupleKey.toString()].delete(deviceId);
   }
 
   /**
@@ -249,6 +247,6 @@ export class NotificationService {
     if (this.deviceIdsToDeliverTo[tuple.toString()] === undefined) {
       return false;
     }
-    return this.deviceIdsToDeliverTo[tuple.toString()].includes(deviceId);
+    return this.deviceIdsToDeliverTo[tuple.toString()].has(deviceId);
   }
 }