Everyone discusses the same few apartments and honestly they're not great for duping - If you're like me and don't care about having an actual apartment then just buy a garage such as 331 Supply St - ($135,000, ten cars). The duping glitch works perfectly here every time, partly because no-one else is around! It's 10 seconds drive from LSC (the one down the ramp) and Ammunation, your cars spawn in a layby at the edge of the road, far better than getting raped by other players camping outside every other main apartment in the city and driving up and down ramps. Easy escape if being chased too!
The only thing I haven't done here is the blocked door glitch as you actually have two garage doors and the car can still spawn even when they're blocked, just a few yards further down the road, I'll try it with a bus at some point!