diff --git a/FAQ.md b/FAQ.md
index 039847c57c..a2d70b2c14 100644
--- a/FAQ.md
+++ b/FAQ.md
@@ -1055,6 +1055,7 @@ Too large messages and triggering the spam filter of an email server are the mos
 * The attachment size limit for Yahoo [is 25 MB](https://help.yahoo.com/kb/SLN5673.html)
 * *554 5.7.1 Service unavailable; Client host xxx.xxx.xxx.xxx blocked*, please [see here](https://docs.gandi.net/en/gandimail/faq/error_types/554_5_7_1_service_unavailable.html)
 * *501 Syntax error - line too long* is often caused by using a long Autocrypt header
+* *503 5.5.0 Recipient already specified* mostly means that an address is being used both as TO and CC address
 
 **Gmail errors**
 
diff --git a/app/src/main/java/eu/faircode/email/FragmentCompose.java b/app/src/main/java/eu/faircode/email/FragmentCompose.java
index d56608dc27..a52b846550 100644
--- a/app/src/main/java/eu/faircode/email/FragmentCompose.java
+++ b/app/src/main/java/eu/faircode/email/FragmentCompose.java
@@ -4502,6 +4502,23 @@ public class FragmentCompose extends FragmentBase {
                                 checkAddress(ato, context);
                                 checkAddress(acc, context);
                                 checkAddress(abcc, context);
+
+                                List<String> all = new ArrayList<>();
+                                List<String> dup = new ArrayList<>();
+                                for (InternetAddress a : Helper.concat(Helper.concat(ato, acc), abcc)) {
+                                    String email = a.getAddress();
+                                    if (TextUtils.isEmpty(email))
+                                        continue;
+                                    if (all.contains(a.getAddress()))
+                                        dup.add(email);
+                                    else
+                                        all.add(email);
+                                }
+
+                                if (dup.size() > 0)
+                                    throw new AddressException(context.getString(
+                                            R.string.title_address_duplicate,
+                                            TextUtils.join(", ", dup)));
                             } catch (AddressException ex) {
                                 args.putString("address_error", ex.getMessage());
                             }
diff --git a/app/src/main/java/eu/faircode/email/Helper.java b/app/src/main/java/eu/faircode/email/Helper.java
index 674e327cfd..8d309e62c5 100644
--- a/app/src/main/java/eu/faircode/email/Helper.java
+++ b/app/src/main/java/eu/faircode/email/Helper.java
@@ -1564,6 +1564,17 @@ public class Helper {
         return result;
     }
 
+    @SuppressWarnings("unchecked")
+    public static <T> T[] concat(T[] a, T[] b) {
+        if (a == null)
+            a = (T[]) new Object[0];
+        if (b == null)
+            b = (T[]) new Object[0];
+        T[] both = Arrays.copyOf(a, a.length + b.length);
+        System.arraycopy(b, 0, both, a.length, b.length);
+        return both;
+    }
+
     static boolean equal(String[] a1, String[] a2) {
         if (a1 == null && a2 == null)
             return true;
diff --git a/app/src/main/java/eu/faircode/email/MessageHelper.java b/app/src/main/java/eu/faircode/email/MessageHelper.java
index 20ce03fd17..c32bfa4e3d 100644
--- a/app/src/main/java/eu/faircode/email/MessageHelper.java
+++ b/app/src/main/java/eu/faircode/email/MessageHelper.java
@@ -487,7 +487,7 @@ public class MessageHelper {
         if (existing != null)
             result.addAll(Arrays.asList(existing));
 
-        Address[] all = imessage.getAllRecipients();
+        Address[] all = imessage.getAllRecipients(); // to, cc, bcc
         Address[] addresses = convertAddress(InternetAddress.parse(email), identity);
         for (Address address : addresses) {
             boolean found = false;
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index 747c0ab21f..e2e43115eb 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -664,6 +664,7 @@
     <string name="title_no_name">Name missing</string>
     <string name="title_no_email">Email address missing</string>
     <string name="title_email_invalid">Email address invalid: \'%1$s\'</string>
+    <string name="title_address_duplicate">Address \'%1$s\' duplicate</string>
     <string name="title_address_parse_error">Address \'%1$s\' invalid: %2$s</string>
     <string name="title_no_account">Account missing</string>
     <string name="title_no_host">Host name missing</string>