Visio Masters have a time stamp, to determine if Masters are identical

[Visisthebest]

"Each master has an internal timestamp that records the date and time it was last updated. When you drag a master onto the drawing page, Visio uses its timestamp to determine whether to make a copy of the master on the document stencil."

https://flylib.com/books/en/2.332.1.139/1/

[wapperdude]

Nice reference, all-in-all.