The Gemara teaches that Adam was created as a single individual to teach that each person is an entire world, and no two people are alike — not in face, voice, or mind (דעת).
וּלְהַגִּיד גְּדוּלָּתוֹ כּוּ׳. תָּנוּ רַבָּנַן, לְהַגִּיד גְּדוּלָּתוֹ שֶׁל מֶלֶךְ מַלְכֵי הַמְּלָכִים הַקָּדוֹשׁ בָּרוּךְ הוּא, שֶׁאָדָם טוֹבֵעַ כַּמָּה מַטְבְּעוֹת בְּחוֹתָם אֶחָד וְכוּלָּן דּוֹמִין זֶה לְזֶה, אֲבָל הַקָּדוֹשׁ בָּרוּךְ הוּא טוֹבֵעַ כׇּל אָדָם בְּחוֹתָמוֹ שֶׁל אָדָם הָרִאשׁוֹן וְאֵין אֶחָד מֵהֶן דּוֹמֶה לַחֲבֵירוֹ, שֶׁנֶּאֱמַר: ״תִּתְהַפֵּךְ כְּחֹמֶר חוֹתָם וְיִתְיַצְּבוּ כְּמוֹ לְבוּשׁ״. תַּנְיָא, הָיָה רַבִּי מֵאִיר אוֹמֵר: בִּשְׁלֹשָׁה דְּבָרִים אָדָם מִשְׁתַּנֶּה מֵחֲבֵירוֹ: בְּקוֹל, בְּמַרְאֶה, וּבְדַעַת. בְּקוֹל וּבְמַרְאֶה – מִשּׁוּם עֶרְוָה, וּבְדַעַת – מִפְּנֵי הַגַּזְלָנִין וְהַחַמְסָנִין.
The mishna teaches: And this serves to tell of the greatness of the Holy One, Blessed be He, as when a person stamps several coins with one seal, they are all similar to each other. But the supreme King of kings, the Holy One, Blessed be He, stamped all people with the seal of Adam the first man, as all are his offspring, and not one of them is similar to another. The Sages taught in a baraita (Tosefta 8:5): The fact that Adam the first man was created alone serves to declare the greatness of the supreme King of kings, the Holy One, Blessed be He, as a person stamps several coins with one seal, and they are all similar to each other. But the Holy One, Blessed be He, stamps all people with the seal of Adam the first man, and not one of them is similar to another. As it is stated: “It is changed like clay under the seal and they stand as a garment” (Job 38:14). The verse describes people as being created “under the seal,” but their external appearance is different, just as garments can differ in appearance. It is taught in a baraita: Rabbi Meir would say: One person is different from another in three ways: In voice, in appearance, and in thought. The differences in voice and appearance are due to a woman forbidden to him, so that people will not exchange spouses one with another. And the differences in thought are due to the robbers and those who take by force that which is not theirs, as, if everyone thought in a similar way, criminals could take advantage of others because they would understand where they keep their valuables.