If you ever need to combine two identically-named folders together, but they contain a mix of old, new, and duplicate files, it’s likely you will want to be selective in how they are merged together. AppleInsider offers a basic guide to using the Merge option in Finder to keep only the files you want intact.
TLDR: Holding down the option key keeps the newest version of each file at the top level of a folder only. The terminal ‘ditto’ command additionally affects subfolders too.
Once in the Terminal Window, type the Ditto command followed by a space, the path of the source folder you want to merge files from, another space, then the path of the destination folder. Instead of typing the folder paths, they can also be entered by dragging and dropping each folder into the Terminal window.