We've all seen it: someone undercuts us by 1 copper with not just a single post but with two, three, sometimes five-glyph-wide posts. Is there a method to their madness? Maybe. There are certainly advantages and disavantages to posting multiple glyphs while undercutting wars abound.
Most goblins post a single glyph when undercutting. Someone else undercuts us, so we post another to undercut them, creating layers of posts.
~Fewer glyphs are tied up on the AH at any given time.
~Can undercut more often without having to maintain high stocks of glyphs or cancel as often.
~There is the potential that if enough sell the lower boundary price will rise.
~If yours sells and you don't post a fresh one fast enough, your competitor gets a sale at a higher price.
Double-wide (or Triple-, Quadruple-, etc.)
Some folks will post two or more glyphs at a time when undercutting. This is not the same as walling (posting multiple glyphs at a particular price point so that you can either annoy your competition or not have to babysit the auction house).
~If one of your glyphs sell, there is another of yours waiting to be sold.
~It can be used to intimidate your competition by showing off how much more supply you have.
~More of your glyphs are tied up while on the AH.
~You increase the amount of time you're waiting at the mailbox when your glyphs have expired.
~Unless sales are pouring in, the lower boundary price has a smaller opportunity for rising.
What is good when?
For constant undercutting, I personally stick to single-wide posting. Undercutting remains fierce enough on my server that being able to undercut more often and possibly lose a sale in between is better than spending so much time canceling and waiting for the mailbox to refresh. Before I go to bed, however, I will always post a double-wide undercut. If I'm undercut after I log out for the evening, there is nothing I can do anyhow, but I have an extra layer in case I do manage to get some sales overnight.
If you have more control over your server's market or your competition is less vicious in undercutting, constant double-wide undercuts may work for you. If you're stuck fighting against a bot... I'm so sorry.