How do you depict desperate repetition?

Tread lightly. Get your point across but leave the rest to the actors. They will figure out and most likely take the necessary improv steps to convince the audience the emotion is authentic.

For example, in the real world- a woman may scream: "GOD WHY?! NO PLEASE!" two dozen times, but you really don't need to have 3 pages of her screaming this.

So in short, get your point across, keep it realistic but don't harp.

Jane stumbles her way through an explanation in broken cadence.

JANE
No... I mean...no... what? No...

(also a lame example, but that's what I'd do. Toss in a couple of repeated words and let the actor know they're not completing sentences as they speak. Leave the rest to them and the director.)

What you're aiming for is the feel for it. Try to find examples in films that emulate what you're shooting for then find the script. You'll see what it started as and where it went to from there. But really what you want to capture is the essence. If it's a crazy man who has a gun you might handle it like:

ANGIE
Jake, it's going to be okay, just give me the gun.

JAKE
(quietly)
No.

Jake paces, growing more anxious.

ANGIE
You want us dead? Like those other two people you shot?

JAKE
I didn't--

Jake's eyes twitch as he looks at the dead shot up corpses in the corner. Angie's skin shines from the sweat, her hands quiver.

ANGIE
Jake, listen to me.

JAKE
No.

ANGIE
Give me the gun, before anyone else is hurt, I can get help.

JAKE
No, no, no, NO, NO!

Jake raises the gun and shoots her three times. She falls over dead. Jake looks at her, shocked.

JAKE
Angie?

Jake falls to his knees and cries.

hey...

you know what? I read an interview of the pro screenwriter. he said," if you write something like.

the horses run across the screen. a battle ensues for 5 minutes.

that is not going to work. if you want a 5 minute battle scene then you better write the description for 5 minutes.

give attention to details as the devil is in details.

regards,
Ace.Inc1

Sometimes you can use a montage with some sort of vehicle showing the passage of time. I once used a woman building bullietin boards for various holidays to show the passage of time with her alternately saying no everytime my charecter asked her out for a date. Months passed that way effectively. Get creative. it can be done.

There's no reason not to gloss it into acdtion lines.

There's a picture called "Big White" with Robin Williams where his Turrette's Syndrome wife babbles on, repeating, "I know you are but what am I?" staccato, over and over, driving her kidnappers nuts.
Now if you think they wrote that as dialog, you're in trouble.

Instead of something like:

Mindy monotonously jabbers, "I know you are but what am I?" until Moe slaps tape over her mouth.

As with many concerns, once you realize you can break out of the normal format and use direction lines to get things across instead of dialogue or parentheses, it opens up a little space for you.

Never gloss dialogue. Write what you want people to say. It ain't rocket science.

My brother is in film school, and one of the best pieces of advice he's received was to get your hands on a copy of a script and then go through the script as you watch the movie.