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We demonstrate the existence of a novel superconducting state in high quality two-component MgB 2 single crystalline superconductors where a unique combination of both type-1 (λ 1/ξ 1< 1/2) and type-2 (λ 2/ξ 2> 1/2) superconductor conditions is realized for the two components of the order parameter. This condition leads to a vortex-vortex interaction attractive at long distances and repulsive at short distances, which stabilizes unconventional stripe-and gossamerlike vortex patterns that we have visualized in this type-1.5 superconductor using Bitter decoration and also reproduced in numerical simulations.