If a note contains a single '**bold**', what is the realistic alternative explanation? There is no language in which to use double asterisks, and even if it is the case that the user wants to highlight the text with these extra characters instead of that it renders bold, this case is much rarer than users who would want to see bold, in addition, the user who would like to see their asterisks being displayed could alternatively use only a single one, (*text*), since those who would like to see the display of bold would have no alternative.
The requirement that there is more than 1 type of markdown for it to be rendered assumes that a single Markdown pattern may accidentally appear frequently enough to be something to worry about, however when you try to build a realistic example of this happening, it is surprisingly difficult to imagine, syntax is simply too artificial to appear by chance in normal writing.
A single pattern is not a weak sign, it is requiring two that is just demanding more confidence than the situation actually requires, at the cost of failing all my grades on which someone used Markdown sparingly.
