feat: added existence check to FeedExtension

src/main/kotlin/net/moonleay/rssbot/extensions/FeedExtension.kt

@@ -86,6 +86,19 @@ class FeedExtension : Extension() {
                     }
                     return@action
                 }
+                if (SubscriptionRepository.exists(this.channel.id.value.toLong(), feedName) || SubscriptionRepository.existsWithUrl(this.channel.id.value.toLong(), feedUrl)) {
+                    this.respond {
+                        this.embeds.add(
+                            MessageUtil.getEmbed(
+                                EmbedColor.ERROR,
+                                "Feed already exists",
+                                "The feed \"${feedName}\" already exists",
+                                user.username
+                            )
+                        )
+                    }
+                    return@action
+                }
                 val id = SubscriptionRepository.write(
                     SubscriptionData(
                         0,