diff --git a/tests/session/testSession.cs b/tests/session/testSession.cs
index c885e4c..99517ed 100644
--- a/tests/session/testSession.cs
+++ b/tests/session/testSession.cs
@@ -28,6 +28,35 @@ namespace sessionTests
             Assert.IsTrue(Enumerable.SequenceEqual(newK, session.getLatestPublicKey()));
         }
 
+        [Test]
+        public void TestSessionNoPublicDupes(){
+            byte[] publicK = PublicKeyBox.GenerateKeyPair().PublicKey;
+            byte[] privateK = PublicKeyBox.GenerateKeyPair().PrivateKey;
+            byte[] newK = PublicKeyBox.GenerateKeyPair().PublicKey;
+            Session session = new Session(privateK, publicK, true);
+            session.addPublic(newK, getFutureTime(61));
+            try{
+                session.addPublic(newK, getFutureTime(61));
+            }
+            catch(DuplicatePublicKey){return;}
+            Assert.Fail();
+        }
+
+        [Test]
+        public void TestSessionAddPublicInvalidKey(){
+            byte[] publicK = PublicKeyBox.GenerateKeyPair().PublicKey;
+            byte[] privateK = PublicKeyBox.GenerateKeyPair().PrivateKey;
+            byte[] newK = {3, 5};
+            Session session = new Session(privateK, publicK, true);
+            try{
+                session.addPublic(newK, getFutureTime(61));
+            }
+            catch(InvalidKeyLength){
+                return;
+            }
+            Assert.Fail();
+        }
+
         [Test]
         public void TestSessionAddPublicInvalidTime(){
             byte[] publicK = PublicKeyBox.GenerateKeyPair().PublicKey;
diff --git a/treasurechest/chestcrypto/session/exceptions.cs b/treasurechest/chestcrypto/session/exceptions.cs
new file mode 100644
index 0000000..1016897
--- /dev/null
+++ b/treasurechest/chestcrypto/session/exceptions.cs
@@ -0,0 +1,23 @@
+using System;
+namespace chestcrypto{
+
+    namespace exceptions{
+        public class DuplicatePublicKey : Exception
+        {
+            public DuplicatePublicKey()
+            {
+            }
+
+            public DuplicatePublicKey(string message)
+                : base(message)
+            {
+            }
+
+            public DuplicatePublicKey(string message, Exception inner)
+                : base(message, inner)
+            {
+            }
+        }
+    }
+
+}
\ No newline at end of file
diff --git a/treasurechest/chestcrypto/session/session.cs b/treasurechest/chestcrypto/session/session.cs
index 607a76a..1587d10 100644
--- a/treasurechest/chestcrypto/session/session.cs
+++ b/treasurechest/chestcrypto/session/session.cs
@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Linq;
 using System;
 using chestcrypto.exceptions;
 
@@ -18,8 +19,24 @@ namespace chestcrypto{
             private bool strictMode;
             private const int minimumKeyExpireSeconds = 60;
 
+            private void validateKey(byte[] key){
+                if (key.Length != 32){
+                    throw new InvalidKeyLength();
+                }
+            }
+
+            private bool publicKeyExists(byte[] key){
+                foreach( (int, byte[]) k in theirPublicKeys){
+                    if (Enumerable.SequenceEqual(k.Item2, key)){
+                        return true;
+                    }
+                }
+                return false;
+            }
+
             public Session(byte[] masterPrivate, byte[] masterPublic, bool strictMode){
-                if(masterPrivate.Length != 32 | masterPublic.Length != 32){throw new InvalidKeyLength();}
+                validateKey(masterPrivate);
+                validateKey(masterPublic);
                 ourMasterPrivateKey = masterPrivate;
                 theirMasterPublicKey = masterPublic;
                 this.strictMode = strictMode;
@@ -29,6 +46,8 @@ namespace chestcrypto{
             }
 
             public void addPublic(byte[] publicKey, long timestamp){
+                validateKey(publicKey);
+                if (publicKeyExists(publicKey)){throw new DuplicatePublicKey();}
                 if (timestamp < DateTimeOffset.UtcNow.ToUnixTimeSeconds() + minimumKeyExpireSeconds){
                     throw new ArgumentOutOfRangeException();
                 }