diff --git a/app/src/main/java/io/heckel/ntfy/ui/CustomHeaderFragment.kt b/app/src/main/java/io/heckel/ntfy/ui/CustomHeaderFragment.kt
index e18edfcd..78ca57d4 100644
--- a/app/src/main/java/io/heckel/ntfy/ui/CustomHeaderFragment.kt
+++ b/app/src/main/java/io/heckel/ntfy/ui/CustomHeaderFragment.kt
@@ -167,6 +167,8 @@ class CustomHeaderFragment : DialogFragment() {
         // Validate header name and check if a user already exists for this server
         CoroutineScope(Dispatchers.Main).launch {
             var isValid = true
+            val targetBaseUrl = if (header != null) header!!.baseUrl else baseUrl
+            
             if (headerName.isNotEmpty()) {
                 if (!validateHeaderName(headerName)) {
                     headerNameLayout.error = getString(R.string.custom_headers_dialog_error_invalid_name)
@@ -174,20 +176,12 @@ class CustomHeaderFragment : DialogFragment() {
                 } else if (isReservedHeader(headerName)) {
                     headerNameLayout.error = getString(R.string.custom_headers_dialog_error_reserved_name)
                     isValid = false
-                } else if (headerName.equals("Authorization", ignoreCase = true)) {
-                    // Check if a user exists for this server (async)
-                    val targetBaseUrl = if (header != null) header!!.baseUrl else baseUrl
-                    val userExists = if (this@CustomHeaderFragment::repository.isInitialized && validUrl(targetBaseUrl)) {
-                        withContext(Dispatchers.IO) {
-                            repository.getUser(targetBaseUrl) != null
-                        }
-                    } else {
-                        false
-                    }
-                    if (userExists) {
-                        headerNameLayout.error = getString(R.string.custom_headers_dialog_error_user_exists)
-                        isValid = false
-                    }
+                } else if (isDuplicateHeader(targetBaseUrl, headerName)) {
+                    headerNameLayout.error = getString(R.string.custom_headers_dialog_error_duplicate)
+                    isValid = false
+                } else if (headerName.equals("Authorization", ignoreCase = true) && hasUserForServer(targetBaseUrl)) {
+                    headerNameLayout.error = getString(R.string.custom_headers_dialog_error_user_exists)
+                    isValid = false
                 }
             }
             if (header == null) {
@@ -228,6 +222,31 @@ class CustomHeaderFragment : DialogFragment() {
         return reservedHeaders.contains(nameLower) || nameLower.startsWith("sec-websocket-")
     }
 
+    private suspend fun isDuplicateHeader(baseUrl: String, headerName: String): Boolean {
+        if (!this::repository.isInitialized || !validUrl(baseUrl)) {
+            return false
+        }
+        val existingHeaders = withContext(Dispatchers.IO) {
+            repository.getCustomHeadersForServer(baseUrl)
+        }
+        return existingHeaders.any { existingHeader ->
+            // When editing, exclude the current header being edited
+            val isCurrentHeader = header != null && 
+                existingHeader.baseUrl == header!!.baseUrl && 
+                existingHeader.name == header!!.name
+            !isCurrentHeader && existingHeader.name.equals(headerName, ignoreCase = true)
+        }
+    }
+
+    private suspend fun hasUserForServer(baseUrl: String): Boolean {
+        if (!this::repository.isInitialized || !validUrl(baseUrl)) {
+            return false
+        }
+        return withContext(Dispatchers.IO) {
+            repository.getUser(baseUrl) != null
+        }
+    }
+
     companion object {
         const val TAG = "NtfyCustomHeaderFragment"
         private const val BUNDLE_BASE_URL = "baseUrl"
diff --git a/app/src/main/res/values-de/strings.xml b/app/src/main/res/values-de/strings.xml
index 4f0e4cea..ba4212d2 100644
--- a/app/src/main/res/values-de/strings.xml
+++ b/app/src/main/res/values-de/strings.xml
@@ -360,6 +360,7 @@
     <string name="custom_headers_dialog_error_invalid_name">Ungültige Zeichen im Header-Namen</string>
     <string name="custom_headers_dialog_error_reserved_name">Dieser Header ist reserviert und wird von ntfy gesetzt</string>
     <string name="custom_headers_dialog_error_user_exists">Authorization-Header kann nicht hinzugefügt werden, wenn ein Benutzer für diesen Server konfiguriert ist</string>
+    <string name="custom_headers_dialog_error_duplicate">Ein Header mit diesem Namen existiert bereits für diesen Server</string>
     <string name="custom_headers_dialog_name_hint">Header-Name (z.B. CF-Access-Client-Id)</string>
     <string name="custom_headers_dialog_value_hint">Wert (z.B. 9f3c2e4a1b2d4e)</string>
     <string name="custom_headers_dialog_description_add">Einen benutzerdefinierten HTTP-Header hinzufügen, der mit jeder Anfrage an den angegebenen Server gesendet wird.</string>
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index e7f4b874..0531fe03 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -467,8 +467,8 @@
     <string name="user_dialog_button_save">Save</string>
 
     <!-- Custom headers dialog  -->
-    <string name="custom_headers_dialog_title_add">Add Custom Header</string>
-    <string name="custom_headers_dialog_title_edit">Edit Custom Header</string>
+    <string name="custom_headers_dialog_title_add">Add custom header</string>
+    <string name="custom_headers_dialog_title_edit">Edit custom header</string>
     <string name="custom_headers_dialog_base_url_hint">Service URL</string>
     <string name="custom_headers_dialog_name_hint">Header name (e.g. CF-Access-Client-Id)</string>
     <string name="custom_headers_dialog_value_hint">Value (e.g. 9f3c2e4a1b2d4e)</string>
@@ -477,6 +477,7 @@
     <string name="custom_headers_dialog_error_invalid_name">Header name contains invalid characters</string>
     <string name="custom_headers_dialog_error_reserved_name">This header is reserved and set by ntfy</string>
     <string name="custom_headers_dialog_error_user_exists">Cannot add Authorization header if a user is configured for this server</string>
+    <string name="custom_headers_dialog_error_duplicate">A header with this name already exists for this server</string>
     <string name="custom_headers_dialog_button_add">Add</string>
     <string name="custom_headers_dialog_button_save">Save</string>
     <string name="custom_headers_dialog_button_delete">Delete</string>