diff --git a/bitchat/Utils/MessageDeduplicator.swift b/bitchat/Utils/MessageDeduplicator.swift
index 215384fa..66bab3c7 100644
--- a/bitchat/Utils/MessageDeduplicator.swift
+++ b/bitchat/Utils/MessageDeduplicator.swift
@@ -16,6 +16,7 @@ final class MessageDeduplicator {
     private let lock = NSLock()
     private let maxAge: TimeInterval
     private let maxCount: Int
+    private let dateProvider: () -> Date
 
     /// Initialize with default config from TransportConfig
     convenience init() {
@@ -26,9 +27,14 @@ final class MessageDeduplicator {
     }
 
     /// Initialize with custom config for content deduplication
-    init(maxAge: TimeInterval, maxCount: Int) {
+    init(
+        maxAge: TimeInterval,
+        maxCount: Int,
+        dateProvider: @escaping () -> Date = Date.init
+    ) {
         self.maxAge = maxAge
         self.maxCount = maxCount
+        self.dateProvider = dateProvider
     }
 
     /// Check if message is duplicate and add if not.
@@ -38,7 +44,7 @@ final class MessageDeduplicator {
         lock.lock()
         defer { lock.unlock() }
 
-        let now = Date()
+        let now = dateProvider()
         cleanupOldEntries(before: now.addingTimeInterval(-maxAge))
 
         if lookup[id] != nil {
@@ -57,10 +63,13 @@ final class MessageDeduplicator {
         lock.lock()
         defer { lock.unlock() }
 
+        let now = dateProvider()
+        cleanupOldEntries(before: now.addingTimeInterval(-maxAge))
+
         if lookup[id] == nil {
-            let now = Date()
             entries.append(Entry(id: id, timestamp: now))
             lookup[id] = now
+            trimIfNeeded()
         }
     }
 
@@ -106,7 +115,7 @@ final class MessageDeduplicator {
         lock.lock()
         defer { lock.unlock() }
 
-        cleanupOldEntries(before: Date().addingTimeInterval(-maxAge))
+        cleanupOldEntries(before: dateProvider().addingTimeInterval(-maxAge))
 
         // Shrink capacity if significantly oversized
         if entries.capacity > maxCount * 2 && entries.count < maxCount {
diff --git a/bitchatTests/MessageDeduplicatorTests.swift b/bitchatTests/MessageDeduplicatorTests.swift
new file mode 100644
index 00000000..af7d29cc
--- /dev/null
+++ b/bitchatTests/MessageDeduplicatorTests.swift
@@ -0,0 +1,54 @@
+//
+// 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 now = Date(timeIntervalSince1970: 1_000)
+        let deduplicator = MessageDeduplicator(
+            maxAge: 300,
+            maxCount: 4,
+            dateProvider: { now }
+        )
+
+        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() {
+        var now = Date(timeIntervalSince1970: 1_000)
+        let deduplicator = MessageDeduplicator(
+            maxAge: 300,
+            maxCount: 4,
+            dateProvider: { now }
+        )
+
+        deduplicator.markProcessed("expired")
+        now.addTimeInterval(301)
+
+        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"))
+    }
+}