diff --git a/anti-duplication-policy.ts b/anti-duplication-policy.ts
new file mode 100755
index 0000000..d63c312
--- /dev/null
+++ b/anti-duplication-policy.ts
@@ -0,0 +1,71 @@
+#!/usr/bin/env -S deno run
+import { readLines } from 'https://deno.land/std@0.178.0/io/mod.ts';
+import { Keydb } from 'https://deno.land/x/keydb@1.0.0/sqlite.ts';
+
+const ANTI_DUPLICATION_TTL = Number(Deno.env.get('ANTI_DUPLICATION_TTL') || 60000);
+const ANTI_DUPLICATION_MIN_LENGTH = Number(Deno.env.get('ANTI_DUPLICATION_MIN_LENGTH') || 50);
+
+interface InputMessage {
+  type: 'new' | 'lookback';
+  event: Event;
+  receivedAt: number;
+  sourceType: 'IP4' | 'IP6' | 'Import' | 'Stream' | 'Sync';
+  sourceInfo: string;
+}
+
+interface OutputMessage {
+  id: string;
+  action: 'accept' | 'reject' | 'shadowReject';
+  msg: string;
+}
+
+interface Event {
+  id: string;
+  sig: string;
+  kind: number;
+  tags: string[][];
+  pubkey: string;
+  content: string;
+  created_at: number;
+}
+
+/** https://stackoverflow.com/a/8831937 */
+function hashCode(str: string): number {
+  let hash = 0;
+  for (let i = 0, len = str.length; i < len; i++) {
+    const chr = str.charCodeAt(i);
+    hash = (hash << 5) - hash + chr;
+    hash |= 0; // Convert to 32bit integer
+  }
+  return hash;
+}
+
+async function handleMessage(msg: InputMessage): Promise<OutputMessage> {
+  const { kind, content } = msg.event;
+
+  if (kind === 1 && content.length >= ANTI_DUPLICATION_MIN_LENGTH) {
+    const db = new Keydb('sqlite:///tmp/strfry-anti-duplication-policy.sqlite3');
+    const hash = String(hashCode(content));
+
+    if (await db.get(hash)) {
+      await db.set(hash, 1, ANTI_DUPLICATION_TTL);
+      return {
+        id: msg.event.id,
+        action: 'shadowReject',
+        msg: `Event rejected due to duplicated content found within the last ${ANTI_DUPLICATION_TTL}ms.`,
+      };
+    }
+
+    await db.set(hash, 1, ANTI_DUPLICATION_TTL);
+  }
+
+  return {
+    id: msg.event.id,
+    action: 'accept',
+    msg: '',
+  };
+}
+
+for await (const line of readLines(Deno.stdin)) {
+  console.log(JSON.stringify(await handleMessage(JSON.parse(line))));
+}