diff --git a/src/app/api/request/route.ts b/src/app/api/request/route.ts
index 5157716..0d3d16c 100644
--- a/src/app/api/request/route.ts
+++ b/src/app/api/request/route.ts
@@ -1,19 +1,35 @@
+import { z } from "zod";
 import { success, failure } from "@/lib/utils/response";
 import { createPaperRequest } from "@/lib/services/paper";
-import type { CreatePaperInputType } from "@/lib/services/paper";
+import { requestExams } from "@/db/paperRequest";
 import { getPostHogClient } from "@/lib/posthog-server";
 import { randomUUID } from "crypto";
 
+const paperRequestInput = z.object({
+  subject: z.string().trim().min(1).max(200),
+  exam: z.enum(requestExams),
+  slot: z.string().trim().min(1).max(20),
+  year: z.string().trim().min(1).max(20),
+});
+
 export async function POST(req: Request) {
   try {
-    const { subject, exam, slot, year } =
-      (await req.json()) as CreatePaperInputType;
+    const body: unknown = await req.json().catch(() => null);
+    const input = paperRequestInput.safeParse(body);
 
-    if (!subject || !exam || !slot || !year) {
-      return failure("All fields are required.", 400);
+    if (!input.success) {
+      return failure(
+        `send a subject, slot and year, and one of ${requestExams.join(", ")} as exam`,
+        400,
+      );
     }
 
-    const newRequest = await createPaperRequest({ subject, exam, slot, year });
+    const { subject, exam, slot, year } = input.data;
+    const { request, alreadyRequested } = await createPaperRequest({ subject, exam, slot, year });
+
+    if (alreadyRequested) {
+      return success({ message: "this paper has already been requested", request });
+    }
 
     try {
       const posthog = getPostHogClient();
@@ -43,7 +59,7 @@ export async function POST(req: Request) {
     }
 
     return success(
-      { message: "Paper request submitted successfully!", request: newRequest },
+      { message: "Paper request submitted successfully!", request },
       "Created",
       201,
     );
diff --git a/src/db/paperRequest.ts b/src/db/paperRequest.ts
index 501f457..9de5a3d 100644
--- a/src/db/paperRequest.ts
+++ b/src/db/paperRequest.ts
@@ -1,5 +1,7 @@
 import mongoose, { Schema, type Document, type Model } from "mongoose";
 
+export const requestExams = ["CAT-1", "CAT-2", "FAT", "Model CAT-1", "Model CAT-2", "Model FAT"] as const;
+
 export interface IPaperRequest extends Document {
   subject: string;
   exam: string;
@@ -10,12 +12,14 @@ export interface IPaperRequest extends Document {
 
 const paperRequestSchema = new Schema<IPaperRequest>({
   subject: { type: String, required: true },
-  exam: { type: String, required: true },
+  exam: { type: String, enum: [...requestExams], required: true },
   slot: { type: String, required: true },
   year: { type: String, required: true },
   createdAt: { type: Date, default: Date.now }
 });
 
+paperRequestSchema.index({ subject: 1, exam: 1, slot: 1, year: 1 }, { unique: true });
+
 const PaperRequest: Model<IPaperRequest> =
   mongoose.models.PaperRequest ??
   mongoose.model<IPaperRequest>("PaperRequest", paperRequestSchema);
diff --git a/src/lib/services/paper.ts b/src/lib/services/paper.ts
index 4eb52f4..e25ed8d 100644
--- a/src/lib/services/paper.ts
+++ b/src/lib/services/paper.ts
@@ -61,7 +61,15 @@ export async function getCourseCounts(){
 
 export async function createPaperRequest({ subject, exam, slot, year} : CreatePaperInputType){
   await connectToDatabase();
-  return await PaperRequest.create({ subject, exam, slot, year });
+  const result = await PaperRequest.findOneAndUpdate(
+    { subject, exam, slot, year },
+    { $setOnInsert: { subject, exam, slot, year } },
+    { upsert: true, new: true, runValidators: true, includeResultMetadata: true },
+  );
+  return {
+    request: result.value,
+    alreadyRequested: result.lastErrorObject?.updatedExisting === true,
+  };
 }
 
 export async function getSelectedPapers() {