That is an excellent question. If the older one (Mar Keshisha) died before the younger was born, there was probably a nice age difference between the two, since Mar Keshisha seems to have been a recognized Talmid Chacham before he passed away. However, this may have been what happened, since we find that Rashbam (Bava Basra 141a) records that Rav Chisda's children passed away young (see Tosfos there). According to Tosfos (Bava Basra 7b) who writes that Mar Keshisha was the younger of the two, it is certainly possible that Mar Yenuka passed away before the other Mar was born (and the latter was named after the former).

Nevertheless, we find that the two Mar's asked questions to Rav Ashi together (Pesachim 107b, Bava Metzia 66b, Bava Basra 7b), implying that they were both alive and scholars at the same time. If so, Rashi must mean that Rav Chisda gave the same name to children from two different wives (the first of which passed away not long after giving birth) - which is still pretty strange.

Mordecai Kornfeld

Additional Kollel reply:

(a) See the Teshuvas Divrei Malkiel 3:75 (cited in Razei d'Shabsi Bava basra 7b) who was asked whether someone who has a daughter by the name of Simcha may also call his son Simcha. He cites our Rashi as a proof that in former times, they used to call 2 sons by the same name. (However he writes that it appears from Rashi that at birth they were called by the same name "Mar" and afterwards the titles Keshisha and Yenuka were added).

(b) The Divrei Malkiel explains this practice on the basis of the Midrash Bereishis Rabah 37:7, according to Rabbi Yosi, that the early generations possessed Ru'ach Hakodesh so they named their children after events that would take place later. In contrast, later generations did not possess Ru'ach Hakodesh, so they had to call their children after the names of their forefathers. The Divrei Malkiel writes that at times when children were named after events, they would sometimes call 2 sons by the same name, if this was applicable according to the circumstances. In contrast, during generations when sons were named after their ancestors, they would not give 2 sons the same name, because this would give the impression that there were 2 different fathers. The Divrei Malkiel writes that Rav Chisda gave his 2 sons the same name because he possessed Ru'ach Hakodesh, or he knew through a dream [both of these methods were used by the Amora'im] what experiences his sons would go through, and decided to call them both "Mar".

(c) The Divrei Malkiel adds that according to Raban Shimon ben Gamliel in the above Midrash, the early generations knew their Yichus, so they were able to name their children after events. In contrast, the later generations did not know their pedigree so they had to name their children after their ancestors. Rav Chisda lived in Bavel, and since the Gemara in Kidushin 71a states that Bavel had a better Yichus than Eretz Yisrael, it follows that since Rav Chisda possessed a strong Yichus, he was able to allow himself to name his sons after the events that would occur.

(d) The Divrei Malkiel writes further that when 2 sons are given the same name at birth, it is not advisable to leave it like that but rather one should add on later an extra name to be able to distinguish between them, which is why one was called Mar Keshisha and the other Mar Yenuka. The reason for this is similar to the reasoning behind the Will of Rebbi Yehudah ha'Chasid, not to marry a woman with the same name as one's mother etc. because one may confuse different members of the family. Similarly if 2 brothers have the same name, and they are in their parents' house together with their wives, the wives might get mixed up and be with the wrong man.

(e) The Divrei Malkiel concludes that nowadays since we do not normally change names after birth (except in case of illness) one cannot bring a proof from our Rashi and therefore one should not give 2 sons the same name. Similarly one should not call his son Simcha if he already has a daughter Simcha. In fact the Divrei Malkiel expresses disapproval at the practice of naming a girl with a boy's name or vice versa, and writes that this is similar to a man wearing a woman's dress.

[See also the commentary ascribed to Rashi to Divrei ha'Yamim 1 3:6 where one sees that if one son died, one may call a son born afterwards by the same name. See also Adnei Paz #25, #34, cited by the Divrei Malkiel.]

Chanukah Sameach

Dovid Bloom