diff --git a/handshake_messages.go b/handshake_messages.go
index bcd9a83..476a441 100644
--- a/handshake_messages.go
+++ b/handshake_messages.go
@@ -1056,6 +1056,7 @@ func (m *encryptedExtensionsMsg) unmarshal(data []byte) bool {
 		return false
 	}
 
+	seenExts := make(map[uint16]bool)
 	for !extensions.Empty() {
 		var extension uint16
 		var extData cryptobyte.String
@@ -1064,6 +1065,11 @@ func (m *encryptedExtensionsMsg) unmarshal(data []byte) bool {
 			return false
 		}
 
+		if seenExts[extension] {
+			return false
+		}
+		seenExts[extension] = true
+
 		switch extension {
 		case extensionALPN:
 			var protoList cryptobyte.String