Merriam-Webster says "län-ˈje-və-tē" (so a regular "n" sound followed by a soft "g"), but I guess I pronounce it like "long-gevity" (with an "ng" sound followed by a soft "g"). I'm not really pronouncing the "g" twice, because there isn't actually a "g" in the "ng" sound—it's just a nasal sound pronounced in the same place as a hard "g". But it makes sense why someone would think of that as pronouncing the "g" twice, since most people think of pronunciation in terms of spelling (which is why people also talk about "'g' dropping" in words like "singin'").