It looks like this phrase means two persons wear the same or similar outfit, which is very embarrasing. But the word "uniform" seems not to serve such purpose because uniform doesn't make the wearers embarrassing.
撞衫 often used to describe movie stars, celebrity's who just happen to wear the same outfit
'撞' in '撞衫' is the same one in '撞期' (scheduling conflict).
'撞期'(date clash) means two events scheduled at the same date, creating a scheduling conflict between them. It is "(two scheduled events) dates run into each other" in English.
'撞衫' (dress clash) means two people wearing identical outfits at the same event, creating a fashion faux pas for both. It is "(two people) dress the same" in English.
I suggest simply translate 撞衫 as "dress the same" or "identically dress"
"安和瑪麗在頒獎典禮上撞了衫" (Ann and Mary dressed the same at the award show);
"穿成衣可能導致與某人撞衫" (wear ready-to-wear dress many resulted in dressing the same with someone)