diff --git a/bitchat/Utils/MessageDeduplicator.swift b/bitchat/Utils/MessageDeduplicator.swift
index 215384fa..d2fc489d 100644
--- a/bitchat/Utils/MessageDeduplicator.swift
+++ b/bitchat/Utils/MessageDeduplicator.swift
@@ -57,10 +57,13 @@ final class MessageDeduplicator {
         lock.lock()
         defer { lock.unlock() }
 
+        let now = Date()
+        cleanupOldEntries(before: now.addingTimeInterval(-maxAge))
+
         if lookup[id] == nil {
-            let now = Date()
             entries.append(Entry(id: id, timestamp: now))
             lookup[id] = now
+            trimIfNeeded()
         }
     }
 
diff --git a/bitchatTests/MessageDeduplicatorTests.swift b/bitchatTests/MessageDeduplicatorTests.swift
new file mode 100644
index 00000000..6db1117d
--- /dev/null
+++ b/bitchatTests/MessageDeduplicatorTests.swift
@@ -0,0 +1,44 @@
+//
+// MessageDeduplicatorTests.swift
+// bitchatTests
+//
+// Tests for MessageDeduplicator.
+// This is free and unencumbered software released into the public domain.
+//
+
+import Foundation
+import Testing
+@testable import bitchat
+
+@Suite("Message Deduplicator")
+struct MessageDeduplicatorTests {
+    @Test func markProcessed_enforcesMaximumCount() {
+        let deduplicator = MessageDeduplicator(maxAge: 300, maxCount: 4)
+
+        for id in ["a", "b", "c", "d", "e"] {
+            deduplicator.markProcessed(id)
+        }
+
+        #expect(!deduplicator.contains("a"))
+        #expect(!deduplicator.contains("b"))
+        #expect(deduplicator.contains("c"))
+        #expect(deduplicator.contains("d"))
+        #expect(deduplicator.contains("e"))
+    }
+
+    @Test func markProcessed_cleansExpiredEntriesBeforeCountTrim() async {
+        let deduplicator = MessageDeduplicator(maxAge: 0.1, maxCount: 4)
+
+        deduplicator.markProcessed("expired")
+        try? await Task.sleep(nanoseconds: 200_000_000)
+
+        for id in ["b", "c", "d", "e"] {
+            deduplicator.markProcessed(id)
+        }
+
+        #expect(deduplicator.contains("b"))
+        #expect(deduplicator.contains("c"))
+        #expect(deduplicator.contains("d"))
+        #expect(deduplicator.contains("e"))
+    }
+}