Song Dynasty (belonging to the serif family, also known as the serif family) is generally a single line (one horizontal). Bold (belonging to Sans serif family, also known as Sans family), generally double lines (two horizontal lines). Both the single line and the double line represent RMB.

In most national standards printed in Songti, it will be displayed as a single line. In most screen displays based on black body, it will be displayed as a double line. For handwriting, because ordinary handwriting fonts belong to sans serif family, double lines should be used. A related example is that the dollar sign $ is sometimes written as a single line and sometimes as a double line. My opinion is still: both single line and double line make sense, and both represent the same symbol.