A more thorough explanation:

Obviousness-type double patenting is a situation where an inventor tries to obtain a patent for an invention that is only a minor variation of another invention they have already patented or have a pending patent application for. This type of double patenting is not allowed because it is considered unfair to grant multiple patents for the same invention or a minor variation of it.

For example, if an inventor has already obtained a patent for a new type of smartphone, they cannot obtain another patent for a slightly modified version of the same smartphone. This is because the modification is not significant enough to warrant a new patent.

Another example is if an inventor has a patent for a new type of car engine, they cannot obtain another patent for a slightly modified version of the same engine. This is because the modification is not significant enough to warrant a new patent.

Overall, obviousness-type double patenting is not allowed because it goes against the principle of fairness and discourages innovation by allowing inventors to monopolize an invention or a minor variation of it.