"Since there's only one weld symbol, the other identical plate would be left unwelded."- quote
In the detailing world, if the plate had the same pcMk, the detailer doesn't really need to add a welding symbol to the other identical plate. In today's copy and paste drafting/detailing, it wouldn't take that much time.
...but back when you were adding weld symbols with HB graphite while your drawing was taped down to your board, you limited your pencil strokes to only what was absolutely necessary. My old Chief Draftsman would give you down in the country for putting "too much info" on your shop drawing. He would say "make it so a 5yo can understand it but, keep it simple stupid. He didn't like extra things cluttering up a shop drawing.