A women can wear a cap as long as two things are kept in mind; she is not wearing the cap with the intention of appearing masculine, and wearing the cap does not actually make her appear masculine.
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دلائل:
مشکوٰۃ المصابیح: (باب الترجل، 280/2)
قال رسول اللّٰه ﷺ لعن اللّٰه المتشبھین من الرجال بالنساء والمتشبھات من النساء بالرجال.
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.