If prior DUI convictions exist, a wise and experienced DUI lawyer will absolutely challenge them. A prior conviction must have met due process requirements. The rights to counsel, jury trial and confrontation must have been fulfilled or have been knowingly and understandingly waived.
In cases where prior conviction resulted from a guilty plea, the waiver of rights is often not adequate. If the defendant is pardoned, he/she may not receive an enhancement at sentencing when convicted for another DUI, though some courts have held otherwise.