diff --git a/package.json b/package.json
index a4a149b..6eab32a 100644
--- a/package.json
+++ b/package.json
@@ -8,7 +8,8 @@
     "Hiverness",
     "mscdex",
     "jimmybergman",
-    "zackschuster"
+    "zackschuster",
+    "Yuping Zuo (https://github.com/zypA13510)"
   ],
   "repository": {
     "type": "git",
@@ -16,7 +17,8 @@
   },
   "dependencies": {
     "addressparser": "^0.3.2",
-    "emailjs-mime-codec": "^2.0.7"
+    "emailjs-mime-codec": "^2.0.7",
+    "lodash.unionwith": "^4.6.0"
   },
   "devDependencies": {
     "chai": "^4.1.2",
@@ -37,8 +39,8 @@
   ],
   "main": "email.js",
   "scripts": {
-		"rollup": "rollup -c rollup.config.js && npm run rollup:test",
-		"rollup:test": "npm run test -- --file rollup/email.bundle.test.js",
+    "rollup": "rollup -c rollup.config.js && npm run rollup:test",
+    "rollup:test": "npm run test -- --file rollup/email.bundle.test.js",
     "test": "mocha"
   },
   "license": "MIT",
diff --git a/smtp/client.js b/smtp/client.js
index f16a3fc..2b1f931 100644
--- a/smtp/client.js
+++ b/smtp/client.js
@@ -1,6 +1,9 @@
 const { SMTP, state } = require('./smtp');
 const { Message, create } = require('./message');
 const addressparser = require('addressparser');
+const _ = {
+	unionWith: require('lodash.unionwith'),
+};
 
 class Client {
 	/**
@@ -95,11 +98,11 @@ class Client {
 				};
 
 				if (message.header.cc) {
-					stack.to = stack.to.concat(addressparser(message.header.cc));
+					stack.to = _.unionWith(stack.to, addressparser(message.header.cc), (x, y) => x.address === y.address);
 				}
 
 				if (message.header.bcc) {
-					stack.to = stack.to.concat(addressparser(message.header.bcc));
+					stack.to = _.unionWith(stack.to, addressparser(message.header.bcc), (x, y) => x.address === y.address);
 				}
 
 				if (
diff --git a/yarn.lock b/yarn.lock
index 0490fbc..39ca5bd 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -892,6 +892,11 @@ linkify-it@2.0.3:
   dependencies:
     uc.micro "^1.0.1"
 
+lodash.unionwith@^4.6.0:
+  version "4.6.0"
+  resolved "https://registry.yarnpkg.com/lodash.unionwith/-/lodash.unionwith-4.6.0.tgz#74d140b5ca8146e6c643c3724f5152538d9ac1f0"
+  integrity sha1-dNFAtcqBRubGQ8NyT1FSU42awfA=
+
 lodash@^4.17.4, lodash@^4.17.5, lodash@^4.3.0:
   version "4.17.10"
   resolved "https://registry.yarnpkg.com/lodash/-/lodash-4.17.10.tgz#1b7793cf7259ea38fb3661d4d38b3260af8ae4e7"