Short answer is that assuming that it is redundant might be assuming a narrow definition of the word "prepared", narrower than is common usage.
Ok, lets start with a definition. From Collins:
- to make ready or suitable in advance for a particular purpose or for some use, event, etc: to prepare a meal; to prepare to go.
- to put together using parts or ingredients; compose or construct
- (tr) to equip or outfit, as for an expedition
- (Music, other) (tr) music to soften the impact of (a dissonant note) by the use of preparation
- be prepared (followed by an infinitive) to be willing and able (to do something): I'm not prepared to reveal these figures.
In terms of those definitions, the only definition that explicitly mentions time ("in advance") is the first one. It is possible to use prepare as in definitions 2 and 3, but use it as an adjective, and in those cases, it could be argued that adding "pre" is not redundant. "A pre-prepared lunch" implies that lunch was made in advance of lunchtime, and possibly not by the people who were going to eat it. It means "put together using parts or ingredients in advance".
Similarly, I might use some pre-prepared slides in my presentation: I didn't prepare them along with the rest of the slide show, they were already put together, I just used them. Saying that I "reused" the slides implies that they have been used before. Saying that the slides were "pre-prepared" implies that they were made earlier by someone who might be me or not, and may or may not have been used in a previous presentation.
A definition for prepared as an adjective (Collins):
ready for something that is likely to happen, done or made beforehand
And preprepared as an adjective:
prepared in advance
Selectively(!) combining the adjective definitions, we can say that "preprepared" means:
ready for something that is likely to happen, made ready in advance
So you are not just ready, you deliberately made preparations in advance.
Here's an ngram. Its usage IS increasing, but it's still not a patch on "prepared in advance".
Of course, now I'm going to be as careful of my usage of the word "pre-prepared" as I am of the term "old adage".