Reb Yehudah Hachosid (4910-4977; 1150-1217) writes in his will (tzava’a) very serious things about making a shidduch where the chosson has the same name as his father-in-law[1] or the kallah as her mother-in-law. While many poskim question whether the rules of Reb Yehudah Hachosid were intended for all Jews or only for his descendants,[2] the Tzemach Tzedek quotes the Alter Rebbe that the rule about the names of a kallah and her mother-in-law certainly applies to everyone,[3] while the other name issues we treat as a safek question if they apply to everyone or just to his descendants.[4]

As this is a very interesting prohibition,
Poskim have suggested various reasons for this prohibition: (a) not being able to name a child after that grandparent,[5] (b) compromise of kibud av ve’m when calling the spouse by name,[6] (c) ayin hara by having a bunch of people of the same family within same family .[7]

Accordingly, there would be exceptions to this rule: (a) if the grandparent forgoes having a child named after him , (b) if he refers to his wife differently than how his mother is called. (c) if they don’t live together the ayin hara is minimized.[8] However, other poskim are of the opinion that we cannot say with certainty what the reason is and we must therefore be stringent in all cases.[9]

Another rule was that the two mechutanim (fathers-in-law) should not have the same name (this is less severe for the mechutenestes (mothers-in-law) since their names aren’t written into the ksubah or tnayim).[10] Some hold that it only applies if the names are irregular,[11] and some limit it to when it is the parents who are marrying off the children.[12] From the Rebbe’s letters it is apparent that we should be concerned for this rule as well.[13]
The Tzemach Tzedek rules that not to be concerned when one side has an additional dissimilar name.[14]

Therefore, the practice is for one of the people to add a name, preferably at the beginning of their name.[15] This is usually initiated by receiving an aliya with the new name or making a mi-shebeirach,[16] but most important is that they actually use their new name.[17] Preferably the change should be made before the shidduch is announced or at least 30 days before the writing of the tnayim.[18]