On Nazir 26b we have separate positions by Rav Nachman (not inclusive of ingots), and Rav Nachman bar Yitzchak (not inclusive of stacks of beams). Thus, these are two separate individuals. Elsewhere, in Gittin 31b, there is a famous dispute as to whether stam Rav Nachman is bar Yaakov (Tosafot) or bar Yitzchak (Rashi). Many Gemaras such as this one make it evident that Tosafot is correct.
After all, if you see Clark Kent standing next to Superman, they cannot be the same person!
Rav Aharon Hyman, in Toledot Tannaim vaAmoraim, argues that Rashi never said that, because it is so obviously incorrect. Rather, Tosafot was reacting to an incorrect text of Rashi. Thus he writes (feel free to skip the Hebrew, I’ll summarize below…):
(Note at the bottom of the above image he brought our sugya of Nazir 26b.)
Taking it from the top. Gittin 31b has:
רָבָא וְרַב נַחְמָן בַּר יִצְחָק הֲווֹ יָתְבִי הֲוָה חָלֵיף וְאָזֵיל רַב נַחְמָן בַּר יַעֲקֹב דְּיָתֵיב בְּגוּהַרְקָא דְּדַהֲבָא וּפְרִיס עֲלֵיהּ סַרְבָּלָא דְכַרָּתֵי רָבָא אֲזַל לְגַבֵּיהּ רַב נַחְמָן בַּר יִצְחָק לָא אֲזַל לְגַבֵּיהּ אָמַר דִּלְמָא מֵאִינָשֵׁי דְּבֵי רֵישׁ גָּלוּתָא נִינְהוּ רָבָא צָרִיךְ לְהוּ אֲנָא לָא צְרִיכְנָא לְהוּ
The Gemara also relates that Rava and Rav Naḥman bar Yitzḥak were sitting, and Rav Naḥman bar Ya’akov was passing by them while sitting on a gilt carriage [goharka] and with a green cloak [sarbela] spread over him. Rava went to him, but Rav Naḥman bar Yitzḥak did not go to him. He said: Perhaps they are members of the house of the Exilarch. Rava needs them, but I do not need them.
and Tosafot think that Rashi says that Rav Nachman bar Yitzchak was of the family of the exilarch, pointing to Chullin 124a which speaks of plain Rav Nachman as being of the house of the Exilarch.
כי סליק רב אושעיא אשכחיה ליה לרבי אמי אמרה לשמעתיה קמיה הכי אמר עולא והכי אהדר ליה רב נחמן א"ל ומשום דרב נחמן חתניה דבי נשיאה הוא מזלזל בשמעתיה דר' יוחנן
When Rav Oshaya ascended from Babylonia to Eretz Yisrael he found Rabbi Ami, and he said this halakha before him: This is what Ulla said and this is what Rav Naḥman responded to him. Rav Oshaya said to Rabbi Ami: And just because Rav Naḥman is the son-in-law of the family of the Nasi, can he demean the halakhic statement of Rabbi Yoḥanan?
However, while Tosafot have that girsa in Rashi, Rav Hyman points out that our own girsa in Rashi doesn’t say bar Yitzchak, rather just Rav Nachman:
אנא לא צריכנא להו - דרב נחמן חתניה דנשיאה הוה בהעור והרוטב (חולין דף קכד):
Now, there is some kvetching that needs to be done locally in Gittin. For it was Rava who went to Rav Nachman bar Yaakov who was in a gilt carriage, and Rav Nachman bar Yitzchak who said “I have no need of him”. It is easier to take this as an explanation of why he (bar Yitzchak) doesn’t need him, and this would match the dibbur hamatchil of אנא לא צריכנא להו. It is harder, but possible to explain that Rashi is saying what needing is involved, namely that he (bar Yaakov) is a prominent person. But if you kvetch it so, and observe that that is our girsa in Rashi, we don’t have to attribute to Rashi something that seems so obviously wrong.
Rav Hyman also points to Megillah 28b:
הָהוּא דַּהֲוָה תָּנֵי הִלְכְתָא סִיפְרָא וְסִיפְרֵי וְתוֹסֶפְתָּא וּשְׁכֵיב, אֲתוֹ וַאֲמַרוּ לֵיהּ לְרַב נַחְמָן: לִיסְפְּדֵיהּ מָר! אֲמַר: הֵיכִי נִסְפְּדֵיהּ? הֵי צַנָּא דִּמְלֵי סִיפְרֵי דַּחֲסַר?!
In contrast, there was a certain man who used to study halakha, the Sifra, and the Sifrei, and the Tosefta, and he died. People came and said to Rav Naḥman: Let the Master eulogize him. He said to them: How can I eulogize him? Should I say: Alas, a basket filled with books is lost? This would not be true. Although the man studied many areas of Torah, he was not proficient in them.
תָּא חֲזִי מָה בֵּין תַּקִּיפֵי דְּאַרְעָא דְיִשְׂרָאֵל לַחֲסִידֵי דְבָבֶל.
The Gemara compares the conduct of Reish Lakish in Eretz Yisrael to that of Rav Naḥman in Babylonia. Come and see what the difference is between the harsh scholars of Eretz Yisrael and the saintly ones of Babylonia. Although Reish Lakish was known for his harsh nature, he was still more respectful than Rav Naḥman, who was known for his saintliness.
Rashi there seems to indicate this stam Rav Nachman is Rav Nachman bar Yitzchak:
תקיפי ארעא דישראל - ריש לקיש דאמרינן במסכת יומא (דף ט:) דאפילו בהדי רבה בר בר חנה לא משתעי דמאן דמשתעי בהדי ריש לקיש בשוקא יהבין ליה עיסקא בלא סהדי: רב נחמן בר יצחק מחסידי בבל בשילהי מסכת סוטה (דף מט:) דקאמר ליה לא תתני יראת חטא דאיכא אנא:
pointing to Sotah 49b, where we find stam Rav Nachman mentioned:
מִשֶּׁמֵּת רַבִּי בָּטְלָה עֲנָוָה וְיִרְאַת חֵטְא אֲמַר לֵיהּ רַב יוֹסֵף לְתַנָּא לָא תִּיתְנֵי עֲנָוָה דְּאִיכָּא אֲנָא אֲמַר לֵיהּ רַב נַחְמָן לְתַנָּא לָא תִּיתְנֵי יִרְאַת חֵטְא דְּאִיכָּא אֲנָא
The final line of the mishna states that from the time when Rabbi Yehuda HaNasi died, humility and fear of sin ceased. Rav Yosef said to the tanna who reviewed the mishna: Do not teach that humility ceased, for there is still one who is humble, namely me. Rav Naḥman similarly said to the tanna who reviewed the mishna: Do not teach that fear of sin ceased, for there is still one who fears sin, namely me.
However, Rav Hyman suggests that Rashi wrote RNbY, meaning bar Yaakov, and this was erroneously expanded into bar Yitzchak.