diff --git a/system/ssh_key.go b/system/ssh_key.go
index a75896c..5a76076 100644
--- a/system/ssh_key.go
+++ b/system/ssh_key.go
@@ -15,11 +15,65 @@
 package system
 
 import (
+	"bufio"
 	"fmt"
 	"os"
 	"strings"
 )
 
+func diffLines(src, dst []string) []string {
+	var tgt []string
+
+	mb := map[string]bool{}
+
+	for _, x := range src {
+		mb[x] = true
+	}
+
+	for _, x := range dst {
+		if _, ok := mb[x]; !ok {
+			mb[x] = true
+		}
+	}
+
+	for k, _ := range mb {
+		tgt = append(tgt, k)
+	}
+
+	return tgt
+}
+
+func readLines(path string) ([]string, error) {
+	var lines []string
+
+	file, err := os.Open(path)
+	if err != nil {
+		return lines, err
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		lines = append(lines, scanner.Text())
+	}
+	return lines, scanner.Err()
+}
+
+// writeLines writes the lines to the given file.
+func writeLines(lines []string, path string) error {
+	file, err := os.Create(path)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	w := bufio.NewWriter(file)
+	for _, line := range lines {
+		fmt.Fprintln(w, line)
+	}
+	return w.Flush()
+}
+
 // Add the provide SSH public key to the core user's list of
 // authorized keys
 func AuthorizeSSHKeys(user string, keysName string, keys []string) error {
@@ -29,7 +83,7 @@ func AuthorizeSSHKeys(user string, keysName string, keys []string) error {
 
 	// join all keys with newlines, ensuring the resulting string
 	// also ends with a newline
-	joined := fmt.Sprintf("%s\n", strings.Join(keys, "\n"))
+	// joined := fmt.Sprintf("%s\n", strings.Join(keys, "\n"))
 
 	home, err := UserHome(user)
 	if err != nil {
@@ -43,12 +97,12 @@ func AuthorizeSSHKeys(user string, keysName string, keys []string) error {
 	}
 
 	authorized_file := fmt.Sprintf("%s/.ssh/authorized_keys", home)
-	f, err := os.OpenFile(authorized_file, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
-	if err != nil {
-		return err
+	var newkeys []string
+	for _, x := range keys {
+		newkeys = append(newkeys, strings.Split(x, "\n")...)
 	}
-	defer f.Close()
-	_, err = f.WriteString(joined)
+	oldkeys, _ := readLines(authorized_file)
 
-	return err
+	diffkeys := diffLines(oldkeys, newkeys)
+	return writeLines(diffkeys, authorized_file)
 }