The spec of DVD players are required to play dual-layer discs. Therefore, there are only few DVD players that are not able to read dual-layer DVDs. Even if they don't, it supposed to be malfunction and you should probably send them back to manufacturer. However, some discs claim that they can do "seamless layer change" and this technique is go beyound the regular DVD specs. So, some of the DVD players cannot play them.
As for double-sided DVD, you can use almost any DVD players to play it as long as you swich the disc yourself.