I attended Kevin Mandeville's workshop, Progressive Enhancement & Interactive Email, at last year's Litmus Conference in Boston where I learned about his elegant solution for preventing double bolding. The hack is below (font stacks may differ for you, obviously).

Recently however, I noticed a really strange bug with this hack. Outlook 2011 & Outlook 2016 (both for Mac), and the iPad Mini (iOS9) double-bold, ignoring the hack. Anyone else notice this? A Fix perhaps?

    @media screen and (-webkit-min-device-pixel-ratio: 0), screen and (min--mozdevice-
    pixel-ratio: 0) {
    #font-weight-normal {
    font-weight: normal !important;

    <table border="0" cellpadding="0" cellspacing="0" width="100%">
    <td align=“center” style="color: #444e4f; font-family: 'proxima_nova_rgbold', Proxima
    Nova, Helvetica, Arial, sans-serif; font-size: 28px; line-height: 36px; font-weight: bold;"
    The Email Design Conference