1. A method for performing an optical proximity correction (OPC) process on a layout by incorporating a critical dimension (CD) correction of a structure therein, the layout requiring double exposures in fabrication, the method comprising: separating the layout into a first portion and a second portion corresponding to the two exposures using a computer; creating a model for calculating a CD correction for a site on the first portion, the model corresponding to a topography change on the site due to the double exposures; implementing an OPC iteration for the site based on the model to generate an OPC solution for the first portion using the computer; combining the OPC solution for the first portion with an OPC solution for the second portion to generate an OPC solution for the layout to generate a mask for fabricating a structure using the layout using the computer; designing a multiple levels OPC test structure and sampling plan corresponding to multiple sources of CD changes using the computer; and detecting a CD change under each level of the multiple levels OPC test structure and sampling plan using the computer.