From http://www.shemayisrael.com/parsha/alport/archives/matos67.htm
ונבח הלך וילכד את קנת ואת בנתיה ויקרא לה נבח בשמו 32:41-42
Rav Aizik Ausband was once faced with a dilemma. His father-in-law,
Rav Avrohom Yitzchok Bloch Hy”d, was one of the leaders of the Telz
yeshiva who was tragically murdered in the Holocaust. Rav Ausband’s
wife was pregnant, and if the baby was a boy, he wished to commemorate
the memory of his father-in-law by naming the baby Avrohom Yitzchok.
The problem was that Rav Ausband’s full name is R’ Yitzchok Aizik.
Since the prevalent custom is not to give a child the same name as his
parents, Rav Ausband wondered whether he was permitted to have a son
named Avrohom Yitzchok. Should this be avoided because both names
would contain “Yitzchok,” or does the fact that each would have an
additional name make it acceptable?
Rav Ausband presented his query to Rav Eliezer Silver, who replied
that the Torah “explicitly” answers this very question at the end of
Parshas Matos. Yair conquered the villages in Gilad and renamed them
Chavos-Yair – the villages of Yair. Rashi explains that because Yair
had no children, he named the villages after himself to memorialize
his name.
The Torah continues and recounts that Novach captured K’nas and its
suburbs and renamed them Novach in his name. Why isn’t the expression
“in his name” also used in conjunction with Yair naming his villages
Chavos-Yair? We even find later (Devorim 3:14) that Moshe mentioned
that Yair called the cities על שמו – after his name.
Rav Silver answered that because Novach gave his exact name to his
conquered territory, the Torah says that he called them “in his name.”
Yair, on the other hand, added an additional name in calling his
villages not “Yair” but “Chavos-Yair.” Moshe considered this a
memorial to Yair’s name, but the additional name makes it a new name
which can’t be considered “in his name.” As a result, the names
Yitzchok Aizik and Avrohom Yitzchok, each of which contains an
additional name, are considered two different names and may be used by
a father and son!
Perhaps in this case, the woman should name the son Shimon with another name to avoid the issue. Alternatively, if the husband already has a middle name, she can name the son just Shimon.