The 90 days you did in January will only take you up to the extension date.
The system won't allow you to extend online once you apply on 21 February, if that is the date your extension is due.
When applying for your extension, the immigration officer will provide you with an 90 day extension once your stamp is put in your passport.
I came across this and was as well when I received the same notice, I was a little confused at 1st, e.g. extension was due on 31 January, I tried to the 90 day extension on line just beforehand after the email reminder, but it wouldn't allow me to renew it.
The above said, it would appear that the system knows when your extension is due, but still sends out the 90 day email renewal notice, for whatever reason, maybe it's there way to say, oi, time to renew your extension ?
When I did my extension (marriage), they said that I didn't require a 90 day extension until my stamp went into my passport on 1 March, so it would be done then.
I didn't like the fact that I was without a 90 day extension, but immigration assured me it was ok, because my extension was under consideration, hence no further 90 day extension was required.
I look forward to the day they do away with this 90 day extension and just do annual extensions, albeit it 5 year extensions would be a dream...LOL
Hope the above clears things up for you.