Handle when a feed in a folder being copied/moved already is subscribed in target account

Mac/MainWindow/Sidebar/SidebarOutlineDataSource.swift

@@ -492,6 +492,18 @@ private extension SidebarOutlineDataSource {
 			case .success(let destinationFolder):
 				let group = DispatchGroup()
 				for feed in folder.topLevelFeeds {
+					if let existingFeed = destinationAccount.existingFeed(withURL: feed.url) {
+						group.enter()
+						destinationAccount.addFeed(existingFeed, to: destinationFolder) { result in
+							group.leave()
+							switch result {
+							case .success:
+								break
+							case .failure(let error):
+								NSApplication.shared.presentError(error)
+							}
+						}
+					} else {
 						group.enter()
 						destinationAccount.createFeed(url: feed.url, name: feed.editedName, container: destinationFolder) { result in
 							group.leave()
@@ -503,11 +515,13 @@ private extension SidebarOutlineDataSource {
 							}
 						}
 					}
+				}
 				group.notify(queue: DispatchQueue.main) {
 					completion()
 				}
 			case .failure(let error):
 				NSApplication.shared.presentError(error)
+				completion()
 			}
 		}