^ I've noticed that too. I usually delete messages right away that I'm not going to reply to, but sometimes I get lazy and leave them there, and a suspiciously large proportion of them become deleted accounts shortly after.
Also, messages that are much more detailed but are not personalised sometimes get sent to me more than once. They tend to be about a paragraph, quite friendly, saying hello and describing what they are looking for (often something a bit different to the norm) and that they'd love to hear from me if I'm interested. A month or two later...oh that same person is sending me the same message again!
Another type of message I receive more than once is one that tells me they enjoyed reading my profile and found me interesting, or that we have a lot in common, or that I sound [insert nice quality], but again it's not personalised - they are actually sending this same message to multiple people. Presumably without paying attention, because they forgot they had already written to me before.