If stress is numerically equal to twice that of young's modulus, the elongation will be

Y=Longitudinal StressLongitudinal strain ⇒Longitudinal Stress=Y×Longitudinal Strain⇒2Y=Y×Longitudinal Strain ⇒2=Longitudinal Strain⇒Δℓℓ=2 ⇒Δℓ=2ℓ