diff --git a/Mac/Preferences/Accounts/AccountsReaderAPIWindowController.swift b/Mac/Preferences/Accounts/AccountsReaderAPIWindowController.swift
index 35ce1c4c6..66a286151 100644
--- a/Mac/Preferences/Accounts/AccountsReaderAPIWindowController.swift
+++ b/Mac/Preferences/Accounts/AccountsReaderAPIWindowController.swift
@@ -91,6 +91,11 @@ class AccountsReaderAPIWindowController: NSWindowController {
 			return
 		}
 		
+		guard !AccountManager.shared.duplicateServiceAccount(type: accountType, username: usernameTextField.stringValue) else {
+			self.errorMessageLabel.stringValue = NSLocalizedString("There is already an account of this type with that username created.", comment: "Duplicate Error")
+			return
+		}
+		
 		let apiURL: URL
 		switch accountType {
 		case .freshRSS: