Rule of first mention?

The rule of fist mention is used by various forms of allegorical interpretation. Rather than inventing an allegorical meaning, clues are taken from the first mention.

For example: Garments are interpreted to represent either righteousness, or works based upon the first mention of garments in Gen 3:21.

Ge 3:7 And the eyes of them both were opened, and they knew that they [were] naked; and they sewed fig leaves together, and made themselves aprons.

Is there a biblical authority for using such a method?

In modern hermeneutic systems, there is no rule to constrain a type or metaphor of first mention to any other usage. Free-for-all allegory is used though despised by the expositor since there are no rules. It often seems that even the best exegetes cannot resist using allegory.

In sensus plenior, since the use of a metaphor must be consistent every where it is used, the first mention is no different in authority than any other, but often contains more clues to unpacking the meaning of the metaphor.

Some suggest that the rule of first mention defines the symbol in such a way that following instances must follow the pattern. But in SP each picture is like a transparency, and when stacked and viewed as a single picture, a more clear picture of Christ is seen.

By definition, in sensus plenior, all men are types are of Christ, so by definition both Moses and Joshua are types of Christ. The question does not revolve around differences, such as having their sandals removed. Even if they did there is more similar, than not. Moses did not have Aaron remove his sandals, he only received a kiss from him. Joshua did not have someone else remove his sandals, so why would we expect that John should be required to remove Jesus's sandals for Moses or Joshua to be a type of Christ?

Just because an author explains his metaphor does not mean that he has changed it. In fact, by the rules of SP, the meaning of the metaphor must sum up an include everything that is said about it. If John had said that a garment meant a new life, rather than righteousness, then we would be compelled by SP to find a solution to the riddle which included both, such as "a new life in God's righteousness". (This doesn't work, because the fig leaf aprons do not represent righteousness of God and the meaning of garments must include the fig leaves.) And often subtle difference actually help define different terms as aspects of a greater unifying idea.

An example where the scriptures changes the typological target is when Moses is called a god to Pharaoh, and Aaron his mouthpiece. Prior to this, Moses represents Jesus. After this he represents God, and Aaron represents Jesus as his mouthpiece.