Gemara

(R. Yochanan): Gechazi was punished because he referred to his Rebbi by his name - "v'Zeh Benah Asher Hecheyah Elisha."

Kidushin 31b (Beraisa): A Chacham changes the name of his father or Rebbi. The one who broadcasts the lesson says the actual names.

When Mar bar Rav Ashi would teach, he would say 'my father, my teacher.' The broadcaster would say 'Rav Ashi'.

Rishonim

Rambam (Hilchos Mamrim 6:3): Fear of a parent... he does not call him by his name in his lifetime or after death. Rather, he says 'my father, my teacher.' If others have the same name as his father, he changes their name when addressing them. It seems that one is careful about this only if it is an atypical name that people are not used to. Names that everyone uses, like Avraham, Yitzchak..., and similarly in every language, there is no problem to call others by this name in his father's absence.

Rambam (Hilchos Talmud Torah 5:5): A Talmid may not call his Rebbi by his name, even not in front of him, if it is a rare name, that everyone will know who it is.

Kesef Mishneh and Beis Yosef (YD 242 DH u'Mah she'Chosav v'Hu): Rashi explains that the Isur is to call his Rebbi by his name without saying Mari (my master) or Rebbi. The Torah supports him; Yehoshua said "Adoni Moshe." It is the Rambam's reasoning to distinguish rare from standard names. He wrote similarly in Hilchos Mamrim. However, there he permits calling only others by a standard name, but not to his father, and only in his father's absence. Here he permits calling his Rebbi (if he has a standard name), and all the more so others with the same name, even in front of his Rebbi! This requires investigation. There he forbids calling even others a rare name in his father's absence. Here he permits this! What is his source to require changing others' names? The Rambam's words in Hilchos Mamrim are more precise. There he wrote that it is his own reasoning.

Drishah (YD 240:4): All his questions are due to his text of the Rambam at the end of Hilchos Mamrim 'not in front of him.' The Tur's text did not have these words, or he deleted them from his text. Also there, the Rambam permits calling others by his father's name unless it is rare and in front of him. In Hilchos Talmud Torah he discusses calling a Rebbi with a rare name, to forbid this even in his absence. A standard name is forbidden only in front of him. The Rambam did not explicitly say so, but surely he means so. A rare name is forbidden in his absence, for it is clear that he discusses his Rebbi. It is as if we heard him call his Rebbi. Gechazi was punished for saying "Elisha". Even though the Kesef Mishneh calls this a standard name, since it was clear whom he referred to ("who revived the boy"), it was a disgrace not to call him 'Rebbi'. I explained why the Tur and Shulchan Aruch omitted 'in his absence.' It is difficult why the Rema added these words.

Bach (5): Kidushin 31b says that a Chacham changes the name of his father or Rebbi. The Rambam explains that this refers to mention of others with the same name. To call his own father or Rebbi by his name is Apikorsus! This is why the Gemara says 'he changes.' It does not say 'he does not mention them by name.' It should not look like he calls his Rebbi by name. This concern applies only to a rare name. The Tur was astounded because he explains like Rashi, that the Chacham changes his father's name. The Rambam forbids to call others by his father's name only in front of his father. In his absence, it is merely proper to refrain.

Taz (242:4): Perhaps the Rambam forbids even a standard name in front of his Rebbi. He discusses a rare name to forbid even in his absence, but in front of him is forbidden in every case! We can say that 'he may not mention...' refers to in front of him, and then even a standard name is forbidden! Why didn't the Kesef Mishneh and Drishah ask why the Rambam permits calling his Rebbi by a standard name in his absence, but forbids regarding a father even after death? A Chacham must change his father's name even in his absence! This is even for a standard name. Rather, the text of the Rambam must say 'he may not call his Rebbi by his name, or mention his name in front of him, or even call others with his name, if it is rare,' like the Shulchan Aruch says. The Ran (Kidushin 13a) says that the Rambam forbids even to call others with his Rebbi's name, if it is rare, even in his Rebbi's absence, but his Rebbi's name is always forbidden.

Poskim

Shulchan Aruch (YD 240:2): He does not call him by his name in his lifetime or after death. Rather, he says 'my father, my teacher.'

Beis Yosef (DH u'Mah she'Chosav Hayah): The Tur was astounded that the Rambam forbids calling others by their name.

Chidushei Hagahos (1): The Beis Yosef did not defend the Rambam, for also he was astounded. Be'er Sheva (below) and Merkeves ha'Mishneh defend the Rambam.

Shulchan Aruch (ibid): If others have the same name as his father, he changes their name when addressing them if it is a rare name that people are not used to.

Gra (7): This is like the Aruch, who explains that Abaye's name was Nachmeni. Rabah bar Nachmeni (who raised Abaye) coined the name Abaye, for he could not call him by his father's name. Others called him Abaye, for (Abaye was Rosh Yeshiva, and) one may not call his Rebbi by his real name. Only Rava, his colleague, used to call him Nachmeni.

Be'er Sheva (Horayos 14a, cited in R. Akiva Eiger): The Aruch says that Rabah did not always call him Nachmeni (even though it is a standard name), i.e. when he was in front of Rabah's father. This is like the Rambam. Rashi (Kidushin 31b) holds that a Chacham changes only his father's name, so he (Gitin 34b) explains that his real name was Abaye, and Rabah called him Nachmeni, like his father.

Rema: If people are used to it, he may call others by that name not in front of his father.

Taz (5): In front of his father even a standard name is forbidden, lest his father or others present think that he calls his father.

Gra (8): Rebbi (the son of R. Shimon ben Gamliel) said 'when I learned from R. Shimon...' There are many such examples. One may not say so to his father; we forbid even after death!

Shulchan Aruch (242:15): One may not call his Rebbi by his name, not in his lifetime or after death. He may not even call others with the same name, if it is a rare name that people are not used to.

Prishah (12): Even standard names are forbidden in front of him. Not in front of him, only a rare name is forbidden, even to call others. The did not question the Rambam here, but in Siman 240 he asked why the Rambam forbids calling others by his father's name, and even in his absence if it is a rare name. There, we must explain the Rambam that way, to avoid a contradiction. Here, a rare name is forbidden in his absence only when discussing his Rebbi, but he may call others by this name. Alternatively, the Tur was astounded about calling others with the same name as his father; here, it is reasonable to forbid this regarding his Rebbi.

Bach (3): The Rambam forbids calling others by the same name only in front of his Rebbi. How can he forbid after death, which is in his absence?! We can say in front of him, it is forbidden to call even others by his Rebbi's name. Not in front of him, there is no Isur, but it shows honor to his Rebbi to change others' names, even after his Rebbi died. To call his Rebbi by his name is forbidden even in his absence. The Shulchan Aruch did not understand to distinguish between what is Asur and what is just improper.

Gra (33): Abaye said 'Mar' to refer to Rabah, even when in front of Rav Yosef, after Rabah died.

Rema: He may mention a name that people are used to, in his absence. This is when he mentions only his name.

Shach (23): He may mention, i.e. call others by this name.

Gra (35): Rebbi said 'Shimon, my son', even though Rebbi's father and Rebbi were named Shimon.

Rema: One may say 'Rebbi Mori Ploni.' (Rashi Sanhedrin 100a.)

Shach (24): This is only in his absence. Darchei Moshe learns from Rashi (100a DH bi'Shmo), who says that Gechazi should have said 'Mori Rebbi Elisha'. This was in his absence. In front of him, one says only Rebbi, without mentioning his name at all. This is our custom.

Gra (36): Similarly, when one says 'Aba' he may mention his father's name, e.g. Aba Chalifta, Yochai Aba.

Yabi'a Omer (2 YD 15:8): Ba'al ha'Itur cited 'Aba Mari Ga'on' (his father's name was Aba). Perhaps he cited a Ga'on called Aba, even though it is a rare name, unlike the Rambam. Or, he is more lenient about writing than speech, unlike the Yam Shel Shlomo, who equates them. I say that the Isur to mention the name of one's father or Rebbi is because their honor is equated to honor of Hash-m. One may write Hash-m's name, but one may not pronounce it.