What is a detailed explanation of the jurisprudential maxim "That which is occupied is not to be preoccupied"?

The meaning of the principle "What is occupied cannot be occupied" is that an eye [or object] already legitimately occupied cannot be occupied by something else. For example, if someone mortgages an item to one person and then mortgages it to another, the right of the first person prevents the establishment of the second person's right, because what is occupied by one thing cannot accommodate another occupation. A fully engaged [or encumbered] liability cannot be further encumbered. This applies to matters of Islamic law, reason, and customary practice.