Fix topic uniqueness issue

ntfysh_client/SubscribeDialog.cs

@@ -5,8 +5,11 @@ namespace ntfysh_client
 {
     public partial class SubscribeDialog : Form
     {
-        public SubscribeDialog()
+        private readonly ListBox _notificationTopics;
+
+        public SubscribeDialog(ListBox notificationTopics)
         {
+            _notificationTopics = notificationTopics;
             InitializeComponent();
         }
 
@@ -30,6 +33,11 @@ namespace ntfysh_client
             return password.Text;
         }
 
+        public string getUniqueString()
+        {
+            return $"{topicId.Text}@{serverUrl.Text}";
+        }
+
         private void button1_Click(object sender, EventArgs e)
         {
             if (topicId.Text.Length < 1)
@@ -64,6 +72,14 @@ namespace ntfysh_client
                 return;
             }
 
+            if (_notificationTopics.Items.Contains(getUniqueString()))
+            {
+                MessageBox.Show($"The specified topic '{topicId.Text}' on the server '{serverUrl.Text}' is already subscribed", "Topic already subscribed", MessageBoxButtons.OK, MessageBoxIcon.Error);
+                DialogResult = DialogResult.None;
+                username.Focus();
+                return;
+            }
+
             DialogResult = DialogResult.OK;
         }