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My mother had a Schengen visa, which she applied for at the Swiss embassy in India. Validity is for 6 months (01-Mar-2013 to 31-Aug-2013) with a single entry visa, She has already spent 88 days in Switzerland and came back to her home country (India), but wants to apply for another Schengen visa at the Swedish embassy in India; validity would be 90 days from 11-Aug-2013 with a single entry visa.
I've heard it's not possible to apply for a new Schengen visa until the old one expires. Is this true? What are her options?