If you don't rush the shift, does it go in smoothly? IE does it only happen if you (fairly rapidly) shift from 1st to 2nd? If you go via neutral, does it help? Does it only happen on the 1st->2nd change, or sometimes on other changes as well? Only at certain road speeds?

It could be any number of things (worn synchros, chipped gear, clutch master cylinder pressure, etc etc). It's fair to say that the problem isn't common.