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To avoid generating the same number twice in a row, you need to keep track of what's happened in the past. How many different images do you have? This expression assumes 10, but just edit the first line to match your number. This is one of those expressions that has to recalculate what's happened on each previous frame, so it might bog down if your comp is fairly long.

nFrames = 10;
seedRandom(index,true);
n = Math.floor(random(nFrames));

f = timeToFrames();

for (i = 0; i < f; i++){
  n += 1 +Math.floor(random(nFrames-1));
  n %= nFrames;
}

framesToTime(n)

Dan

Your English is fine. Maybe it's me that's not articulating very well. :-)

The expression I posted is a time remapping expression that should do what you want. Did you try it?

Dan

Hmmm.... I don't know why it's not working for you. I just tried it and I don't get any repeated frames as far out as I checked (200+ frames or so).

Dan

I just tried it on a 20 second comp (at 29.97 and 25 fps) and I don't get any repeated frames. I guess I'd need to see your project file to figure out why it doesn't work for you.

Dan

Your original expression doesn't work mainly because an expression can't use valueAtTime() to get a value that it calculated previously. The only way an expression can retrieve a previous result for the property to which it is applied is to recalculate it. You can't even apply valueAtTime() to anything other than a property, which is why you got the error message when you tried to apply it to a variable.

Dan