We consider the linear parabolic equation describing the transport of a contaminant in a porous media crossed by a net of infmitly thin fractures. The permeability is very high in the fractures but very low in the porous blocks. We derive the homogenized model corresponding to a net of infmitly thin fractures, by means of the singular measures technique. We prove in a second step that this homogenized model could be obtained indistinctly either by letting the fracture thickness, in the standard double porosity model, tend to zero, or by homogenizing a model with infinitly thin fractures