设向量$\stackrel{\rightharpoonup }{a}$，$\stackrel{\rightharpoonup }{b}$不平行，向量$\lambda \stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}$与$\stackrel{\rightharpoonup }{a}+2\stackrel{\rightharpoonup }{b}$平行，则实数$\lambda$=．