La respuesta correcta es: I just saw somebody with a jacket simar to yours.

I've been following the various comments on this question, and (now that I can finally use my account again!) here's what I wanted to add: You can say either 'I've just seen' or 'I just saw'. This is one situation where the past simple and present perfect really are interchangeable. There's no difference in meaning in this context. And, no, in spite of your doubts, 'with a jacket' isn't Spanglish at all. It's exactly what we'd say in English, too.

I seen someone with the same jacket that you

Lol thanks! My English is native level even though it is not my first language. I'm happy to help.

I guess Ailen did it. Lol