It looks as though Martha was the 2nd Martha to that couple
Birth
JOHNSON, MARTHA mmn ROBINSON
GRO Reference: 1851 June Quarter in SPILSBY Volume 14 Page 623
Death
JOHNSON, MARTHA age 0
GRO Reference: 1851 June Quarter in SPILSBY Volume 14 Page 406
Birth
JOHNSON, MARTHA mmn ROBINSON
GRO Reference: 1855 March Quarter in SPILSBY Volume 07A Page 499