I'm not at the pi at the moment (left the house after making this post) but I can definitely confirm that it erases what I added without appending another.
The game/console I have been using as an example is Game Boy. With the entry for Super Mario Land, the scraped (using default ES scraper) data does not generate a <players> tag in the metadata entry within the gamelist.xml. When I launched the game and played it after adding one, the data saved added a last played date and times played entry, and erased/overwrote the <players>1</players> tag that I added (and saved) prior to launching and playing.
I confirmed by comparing to other titles that my line of xml was formatted exactly as others were that scraped with a <players> tag, but notably those that obtained that data from thegamesdb.net via the built in scraper and remained undisturbed are always greater than 1, which makes me suspect that ES reads a not present/defined value of players as 1? Not sure why it would erase or overwrite it though.