There is a Machlokes between the Minchas Shai,who
says you are supposed to pronounce "Zeicher" with a
Tzeirei and the Heichel Habracha who holds to pronounce
it with a Segol, ," Zecher".
The Minhag is to read it twice, Zecher and Zeicher.
The Ma'aseh Rav (134) writes The Gra originally
read "Zeicher". Later in life he changed
his mind and read Zecher..
It seems, that the Gra never read it twice Zecher and Zeicher