You have added a reference to the output of the project.
In other words, when trying to compile your project, AriaLibrary, to produce AriaLibrary.exe, the compiler imports the assembly AriaLibrary.exe. On disk, this file exists from a previous build.
As such, the compiler finds two of that class, one that it tries to compile now, and one from that previous build, and thus you get the warning.
Since there is no valid reason for having the output a project being imported as a reference to itself, you can safely remove that reference.
Check the references list of the AriaLibrary project and remove the reference with the same name, AriaLibrary.
