If the duvet you have is a summer duvet with a rating of 4.5 TOG it may be worth upgrading to a 10.5 TOG all weather or a 13.5 TOG for winter only.
TOG Rating Guide & Temperature Chart for Babies
Duvet Tog Guide
What is TOG? (& How TOG Rating Can Help Baby Sleep) - The
Sleep Sacks Taking Cara Babies
TOG Ratings for Safe Baby Sleep – Happiest Baby
What is a good tog rating on an underlay? - Ambience Hardwood Flooring
What Is A Tog Rating?
TOG value: Isn't my baby too hot or too cold?
TOG Rating Heat Holders®
What is a TOG rating? – Love to Dream™ NZ
Q&A: What Is A TOG Rating? Everything You Need To Know Explained