No need to sand off the emulsion, although you might want to give it a quick sand to smooth it off.
Emulsion can be used as an undercoat indoors (don't try it outside!) and the gloss applied directly over it, so in your case there is probably no need for an undercoat either.
Real undercoat is better, of course, as it gives a smoother surface on which to put the gloss, but emulsion is okay unless you are painting Buckingham Palace.
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