For the source text click/tap here: Bechorot 49
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When a particular law is stated in the Torah several times, it is not necessarily possible to derive that law from those passages and apply it to other cases through the recognized hermeneutical principles by which the Torah is interpreted.
The Torah has thereby indicated that the law should not be extended to other situations. Were such an extension permissible, the Torah could have stated the law in only one passage, and the second case could have been derived from the first. Since both passages were explicitly written, it follows that the second could not have been derived from the first.
The Tannaim disagree concerning this principle.
The question is not merely whether the Torah repeats itself. It is whether that repetition limits the transfer of law between distinct categories or also prevents the extension of law within a single halakhic institution.
That distinction is particularly relevant to the dispute surrounding the status of the thirtieth day in pidyon ha-ben so we explore the larger halakhic space.