“Why spend money on new things when we can use the old ones?” was probably the mindset of movie makers when they decided to use costumes that had already been used in previous movies. Most films and TV series just don’t have enough money to dress the entire cast in entirely new costumes. And if a dress was especially good (like it was in Titanic) why not take the chance and use it again?
Viral Inspiral has chosen several examples of when characters from different movies wore the exact same clothes. Only the most attentive viewers would have noticed them. Maybe you’re one of them?