צֶ֥דֶק צֶ֖דֶק תִּרְדֹּ֑ף לְמַ֤עַן תִּֽחְיֶה֙ וְיָרַשְׁתָּ֣ אֶת־הָאָ֔רֶץ אֲשֶׁר־יְהֹוָ֥ה אֱלֹהֶ֖יךָ נֹתֵ֥ן לָֽךְ׃
Justice, justice shall you pursue, that you may thrive and occupy the land that the Eternal your God is giving you.
Torah Temimah points to Sanhedrin 32b (or 33b in some editions; the citation is Sanhedrin 32b–33b).
ריש לקיש רמי, כתיב (פ׳ קדושים) בצדק תשפט וכתיב צדק צדק תרדף, הא כיצד, כאן בדין מרומה כאן בדין שאינו מרומה.
English: Reish Lakish raised a contradiction: It is written (in Parashas Kedoshim) “With justice shall you judge,” and it is written “Justice, justice shall you pursue.” How so? Here [the doubled language] refers to a case that is deceptive; there [the single language] refers to a case that is not deceptive.
The doubled “tzedek tzedek” is understood to require extra investigation and scrutiny when the court suspects that the claim or the evidence is fraudulent.
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. The repetition is resolved by distinguishing ordinary cases from cases that appear deceptive.
When a case appears straightforward the court judges with ordinary justice; when the claim or evidence seems deceptive the court must pursue justice with heightened scrutiny and investigation.