To double underline chars (within math enviroment) twice, just use the command \underline two-times.
example
revenue - cost = \underline{\underline{profit}}
$800 - 500 = \underline{\underline{300}}$
If the package ulem is included, the command \uuline could be used:
revenue - cost = \uuline{\underline{profit}}
$800 - 500 = \uuline{\underline{300}}$