This error occurs in the Symantec Endpoint Protection Manager (SEPM), in the "Add Client Install Package" page (Admin > Install Packages > Client Install Packages > Add Client Install Package...).
After trying to add a custom install package, the error "This package already exists." appears.
By default the installation of the Symantec Endpoint Protection Manager console adds two default packages into the database, the 32 bit Symantec Endpoint Protection (SEP) client and the 64 bit SEP client. When new versions of a SEP client are released you will be able to import those new packages. ( Additionally, automatic settings can be created to download & import packages through LiveUpdate.)
These packages can be assign to groups of clients with custom switches to select which components will be installed to the group. The actual client package is not being modified, just instructed to install or not install different components.