diff --git a/.eslintrc.json b/.eslintrc.json
index 3d4c2af8..b62561c6 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -4,6 +4,9 @@
   "plugins": [
     "react"
   ],
+  "env": {
+    "jest": true
+  },
   "rules": {
     "consistent-return": 0,
     "func-names": 0,
diff --git a/lib/utils.js b/lib/utils.js
index 2fed25e4..6e535133 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -1,4 +1,3 @@
-/* eslint prefer-spread: 0 */
 // Inspired by: https://github.com/JedWatson/classnames
 const hasOwn = {}.hasOwnProperty;
 
@@ -8,7 +7,7 @@ export const classNames = (styles, ...args) => {
     if (argType === 'string' || argType === 'number') {
       classes.push(arg);
     } else if (Array.isArray(arg)) {
-      classes.push(classNames.apply(null, arg));
+      classes = classes.concat(arg);
     } else if (argType === 'object') {
       Object.keys(arg).forEach((key) => {
         if (hasOwn.call(arg, key) && arg[key]) {
@@ -18,7 +17,7 @@ export const classNames = (styles, ...args) => {
     }
     return classes;
   }, []);
-  return styleKeys.map(key => styles[key]).join(' ');
+  return [...new Set(styleKeys)].map(key => styles[key]).join(' ');
 };
 
 export const cleanProps = (props, propTypes) =>
diff --git a/lib/utils.test.js b/lib/utils.test.js
new file mode 100644
index 00000000..09595e18
--- /dev/null
+++ b/lib/utils.test.js
@@ -0,0 +1,97 @@
+import { classNames } from './utils';
+
+describe('utils', () => {
+  describe('classNames', () => {
+    it('should calculate string input class ', () => {
+      const styles = {
+        myClass: 'my class',
+      };
+      expect(classNames(styles, 'myClass'))
+        .toBe(styles.myClass);
+    });
+
+    it('should calculate int arg classes', () => {
+      const styles = {
+        1: 'uno',
+      };
+      expect(classNames(styles, 1))
+        .toBe(`${styles[1]}`);
+    });
+
+    it('should calculate multiple arg classes', () => {
+      const styles = {
+        myClass: 'my class',
+        myOtherClass: 'my other class',
+      };
+
+      expect(classNames(styles, 'myClass', 'myOtherClass'))
+        .toBe(`${styles.myClass} ${styles.myOtherClass}`);
+    });
+
+    it('should calculate array of classes', () => {
+      const styles = {
+        myClass: 'my class',
+        myOtherClass: 'my other class',
+      };
+      expect(classNames(styles, ['myClass', 'myOtherClass']))
+        .toBe(`${styles.myClass} ${styles.myOtherClass}`);
+    });
+
+    it('should not duplicate classes', () => {
+      const styles = {
+        myClass: 'my class',
+      };
+      expect(classNames(styles, 'myClass', 'myClass'))
+        .toBe(`${styles.myClass}`);
+    });
+
+    it('should handle object of classes', () => {
+      const styles = {
+        myClass: 'my class',
+        myOtherClass: 'my other class',
+      };
+      const switches = {
+        myClass: true,
+        myOtherClass: true,
+      };
+      expect(classNames(styles, switches))
+        .toBe(`${styles.myClass} ${styles.myOtherClass}`);
+    });
+
+    it('should handle object with some classes switched off', () => {
+      const styles = {
+        myClass: 'my class',
+        myOtherClass: 'my other class',
+      };
+      const switches = {
+        myClass: true,
+        myOtherClass: false,
+      };
+      expect(classNames(styles, switches))
+        .toBe(`${styles.myClass}`);
+    });
+
+    it('should handle multiple input types', () => {
+      const styles = {
+        myClass: 'my class',
+        myOtherClass: 'my other class',
+        1: 'uno',
+        stringClass: 'string class',
+        arrayItemOne: 'array one',
+        arrayItemTwo: 'array two',
+      };
+      const switches = {
+        myClass: true,
+        myOtherClass: true,
+      };
+      expect(classNames(
+        styles,
+        'stringClass',
+        1,
+        ['arrayItemOne', 'arrayItemTwo'],
+        switches,
+       ))
+        .toBe(`${styles.stringClass} ${styles[1]} ${styles.arrayItemOne} ${styles.arrayItemTwo} ${styles.myClass} ${styles.myOtherClass}`);
+    });
+  });
+});
diff --git a/package.json b/package.json
index 6c0594d7..21ba1b14 100644
--- a/package.json
+++ b/package.json
@@ -17,10 +17,10 @@
     },
     "coverageThreshold": {
       "global": {
-        "branches": 93,
+        "branches": 100,
         "functions": 100,
-        "lines": 99,
-        "statements": 99
+        "lines": 100,
+        "statements": 100
       }
     }
   },