Cosmos and Singularity have a lot in common. Singularity however is only a research project to determine the usefulness of pieces that might later be used in .NET and or future versions of Windows. Singularity itself is not intended to ship as a Microsoft supported the operating system. In March 2008 Singularity was released to the public on CodePlex. However the license is for academic use only and thus differs greatly from the goals of Cosmos. Developers of Cosmos should not look at Singularity source to avoid contaminating Cosmos and violating the Singularity license.
If you have looked at Singularity the past, you are welcome to develop on Cosmos however you must be careful not to use your knowledge of Singularity. Unless you were involved deeply into Singularity code this will likely not be a problem. If you are concerned about this, choose purposefully to develop in a different area of functionality in Cosmos.