We have studied the duration of Si-Kα x-ray pulses generated by two subsequent femtosecond-laser pulses (λ=395 nm, pulse duration=180 fs) separated by 20 ps, which were focused on a glass target to intensities of a few 1017 W/cm2. A toroidally bent crystal was used to image the Si-Kα pulses emitted by the target on the cathode of an ultrafast streak camera. While the duration of the first Kα pulse is 1.1 ps, the duration of the second Kα pulse is considerably longer by ≈0.7 ps. This effect is attributed to the preplasma created by the first laser pulse and seen by the second one.