No double-dipping. If you are a retired government employee receiving a pension, then you are no longer allowed to work for the Federal government. If you have a skill that the Federal government needs, then the government can choose from one of the millions currently unemployed to provide that skill. If nobody else has that skill, then you have the option to work for the government at no pay. After all, you are a public servant and it's all about service to your country, right?
Idea No. 5162