- Marty, you must be extremely careful not to run into your other self.
- My other self?
- Yes.

6 seconds sound clip from the Back to the Future Part II movie soundboard.

You can hear this line at 01:13:21.030 in the Blu-ray version of the movie.

Quote context

[...]

- Doc, Doc, come in.

- Marty, what happened to you?

- I went to Biff's house, and you weren't there.

- You must have just missed me. I'm in the back of Biff's car. He's on his way to the Enchantment Under the Sea dance.

- Marty, listen, we may have to abort this entire plan. It's getting much too dangerous.

- Don't worry. The book is on Biff's dashboard. I'll grab it as soon as we get to the school.

- Marty, you must be extremely careful not to run into your other self.

- My other self?

- Yes.

- Remember, your mother is at that exact same dance with you. Yeah.

- Right. This could get heavy, Doc.

- Heavy, heavy.

- Marty, whatever happens, you must not let your other self see you! The consequences could be disastrous.

- Excuse me, sir. Yes, you with the hat.

- Who, me?

- Yes.

[...]