Can New Peel & Stick Tiles Be Applied Over Older Peel & Stick Tiles

If they are clean and level ,yes

Try using a hair dryer set on hot on the old ones. They will show under the new ones if you do not remove them. Another option to reduce the line might be to sand the edges smooth before applying the new ones.

You can, I have done it 2x. It is not the ideal situation, unless you think the tiles and/or glue contain asbestos.

This is what I have experienced. The first time, I was selling a relative's house and the tile was old and stained. I used a type of levelling product for that purpose. It was to help the tile adhere better and filled in the embossed pattern, so it wouldn't eventually show through. It worked beautifully everywhere except on the basement stairwell which was cold. I'm guessing the cold was the problem. I think I ended up using contact cement...I'm not sure why...it was over 15 years ago.

I redid my bathroom tile, and did not use the leveler...big mistake! I also noticed that when we would stand at the vanity brushing our teeth, it felt like we were moving! Sure enough, the tiles were slowly migrating. I ended up using a hairdryer to remove the high traffic ones, and using liquid nails to glue them back down. It has held up.

It is better to remove them if you can. Alternatively, if you dont have to worry about height or doors, you can put a wood underlayment over top the tiles and then do that. We also did that. It was a pain to remove though, when we had our floor professionally redone a few years later.

When you safely can, remove the old junk.

http://homeguides.sfgate.com/installing-vinyl-tile-over-existing-vinyl-floor-43517.html