The double-barred version is not in Unicode because it's just font variations for the same abstract character. Just like the letter g can be written with one or

The double-barred version is not in Unicode because it's just font variations for the same abstract character. Just like the letter g can be written with one or two loops (just look at Helvetica vs Times); Unicode treats it as a font difference. This is called an allograph. Using Baskerville in iOS to render the $ character shows two bars (there are many different variations of Baskerville though).