TT #190 · Parshas Shemini · Aliyah 7 · Passuk 33
Chabad.org — Vayikra Chapter 11 with Rashi
English: “Into its midst”: the four occurrences of the word teach several distinct rules concerning the interior of an earthen vessel, including that the interior of one vessel is not the interior of another.
Chullin 25a.
E-Daf.com — Chullin 25a (Tsurras Hadaf)
Textual analysis — The Chazal derives the stated rule directly from the wording of the verse.
The repeated תוכו creates several distinctions, including תוך תוכו.