You can't get a duplicate!!!
DOL is precluded from sending a duplicate original to anyone other than an immigration or consular officer by 20 C.F.R. Sec. 656.30(e) which states:
Certifying Officers shall issue duplicate labor certifications
only upon the written request of a Consular of Immigration
Officer. Certifying Officers shall issue such duplicate labor
certifications only to the Consular or Immigration Officer who
submitted the written request. An alien, employer, or an
employer or Alien’s agent, therefore may petition an
Immigration or Consular Officer to request a duplicate from the
certifying officer.
The attorney needs to have INS ask DOL for the duplicate. Typically, this can take a LONG time (also, INS usuallly issues an RFE for the original even though they have already been told that they need to request it directly from DOL). There's not much to be done at this point but this is a good reason not to misplace the original LC.