As explained in the article entitled Patent Applications, a patent provides a 20 year monopoly granted by the Federal government. Not surprisingly, you can’t extend the monopoly by patenting the invention twice.
The idea that an invention only is allowed to be patented once, is related to the idea that a patent can cover only one independent and distinct invention. The concept of a single invention for each patent is discussed in the article entitled Restriction Practice. Recall restriction practice allows the examiner to require the applicant to select one invention for prosecution. The un-elected inventions can be subject of a separate “divisional” application having the same filing date (and presumably the same 20 year term).
There are two types of double patenting. The first is termed “statutory double patenting” and is never permitted. Remembering that the patent claims define the patent coverage or scope of invention, two separate patent can’t literally claim the same invention. Changing the claim wording won’t help. If it’s the same invention, it can’t be claimed twice. Note this is different from one patent dominating a separate patent. Recall the patent for the AB combination dominating a later filed patent for the AB&C combination. In that situation, there can be two separate patents, either owned by separate entities or the same entity.
The other type of double patenting is termed “obviousness double patenting” or “non-statutory double patenting”. In this case, the claimed inventions are not identical. The problem is that they are not patentably distinct. (Remember the term patentably distinct is used in restriction practice. A single application is permitted to claim two separate but patentably indistinct inventions. Remember this test is essentially the same as the test for obviousness. In invention must be novel, useful and non-obvious to be patentable. If an application is claiming something that is not patentably distinct from the prior art, that is, it would be an obvious variation to the person skilled in the art, then its not patentable.) The same issue is at work. A single entity can’t have one patent, with one 20 year monopoly, and then make a minor (obvious) change and make that the claim of a separate patent. It is a prohibited attempt to extend the patent term.
As an accommodation to the applicant, the USPTO will allow a separate patent to issue provided the applicant expressly and in writing surrenders the “extra” term of the second patent. In other words, both patents will end on the same (and earlier) date. This is called a “terminal disclaimer”. An important aspect of this is that the same entity owns both issued patents. This common ownership must continue for the duration of the patents or the later patent will be voided.
I have skipped over the issues relating to common ownership existing at the time the inventions were conceived and reduced to practice. However, the law recognizes that two different employees can each make marginal improvements to a product or process and that the employees have a contractual obligation to assign their rights to the same employer. In that circumstances, the USPTO recognizes a single entity for purpose of allowing both patent provided there is a terminal disclaimer and the patents remain under common ownership.