How to automatically copy new photos from an iPhone to a Windows PC when plugged in?
The Windows Photos App solution will be the most efficient for you. It'll offer exactly what you requested and you can be assured that it'll not mix up photos or have duplicated copies since its intelligent enough to label these photos. 

Or you can use the file explorer. 
iMore has a great breakdown of the steps you should take if you want to save your photos. As well as other options.

https://www.imore.com/how-transfer-your-iphone-and-ipad-photos-windows-10#app