Encountered a problem at the studio last weekend.
Was shooting with a 1Ds MkIII body and Elinchrom Digital 3000 powerpack and flashhead, triggered with a pocket wizard.

At long exposures (6 to 10s), the shutter triggered the flash on first curtain sync but subsequently, a second flash was triggered somewhere in the middle of the exposure time. The second flash may have been triggered after the powerpack had recharged.

Does anyone know what the issue is (equipment or settings) or encountered a similar problem?