Don't allow duplicate accounts on launch. Issue #2448

Frameworks/Account/AccountManager.swift

@@ -165,6 +165,15 @@ public final class AccountManager: UnreadCountProvider {
 		NotificationCenter.default.post(name: .UserDidDeleteAccount, object: self, userInfo: userInfo)
 	}
 	
+	public func duplicateServiceAccount(type: AccountType, username: String?) -> Bool {
+		for account in accounts {
+			if account.type == type && username == account.username {
+				return true
+			}
+		}
+		return false
+	}
+	
 	public func existingAccount(with accountID: String) -> Account? {
 		return accountsDictionary[accountID]
 	}
@@ -351,15 +360,23 @@ private extension AccountManager {
 			return
 		}
 		
+		filenames = filenames?.sorted()
+
 		filenames?.forEach { (oneFilename) in
 			guard oneFilename != defaultAccountFolderName else {
 				return
 			}
 			if let oneAccount = loadAccount(oneFilename) {
+				if !duplicateServiceAccount(oneAccount) {
 					accountsDictionary[oneAccount.accountID] = oneAccount
 				}
 			}
 		}
+	}
+	
+	func duplicateServiceAccount(_ account: Account) -> Bool {
+		return duplicateServiceAccount(type: account.type, username: account.username)
+	}
 
 	func sortByName(_ accounts: [Account]) -> [Account] {
 		// LocalAccount is first.