Hi there!
Whether to retake or not really depends on what you want to do
after NS.
If your grades allow you to go to ITE, then you need not
retake.
If you want to go to polytechnic to get your diploma, then yes,
you need to retake. It really depends, you have to check with the
various polytechnics for their minimum entry requirements. Most
poly courses require a at least a D7 for EMaths, its part of the
L1R4.
If you have a N Level, then use your N Level to go to ITE.