You can help expand this article with text translated from the corresponding article in Swedish. (August 2018) Click [show] for important translation instructions. View a machine-translated version of the Swedish article. Machine translation, like DeepL or Google Translate, is a useful starting point for translations, but translators must revise errors as necessary and confirm that the translation is accurate, rather than simply copy-pasting machine-translated text into the English Wikipedia. Do not translate text that appears unreliable or low-quality. If possible, verify the text with references provided in the foreign-language article. You must provide copyright attribution in the edit summary accompanying your translation by providing an interlanguage link to the source of your translation. A model attribution edit summary is Content in this edit is translated from the existing Swedish Wikipedia article at [[:sv:Niclas Gustaf Duncan]]; see its history for attribution. You may also add the template {{Translated|sv|Niclas Gustaf Duncan}} to the talk page. For more guidance, see Wikipedia:Translation.

Niclas Gustaf Duncan (20 November 1711 – 23 April 1771) was a Swedish post official and spy. Between 1743 and 1748, he was active as an agent of Carl Gustaf Tessin to act as a double agent in Swedish service and spy on the Russian diplomat Johann Albrecht Korff under the protect of acting as his spy on Herman Cedercreutz, who was the Swedish envoy in Russia at the time.^[1]