Writing a diary entry and retelling the story.

Well, it sounds complex and maybe tricky to pull off well.

As far as redundancy, just use the diary entry as a teaser; something that would be likely to trigger the flashback to the past. You probably don't need to put the full diary entry in the story. You can indicate in various ways that the part of the diary that you put in the story is just a portion of the actual diary entry.

I notice that when I write journal entries (personal ones, at least), I'm often rather vague and don't go into much detail, because most of the stuff I'm writing is stuff I already know about. In personal journals/diaries, people might not feel the need to explain certain things like locations, family ties, details of things, etc., because these are known to them and they have no need to explain them to themselves. (For example, in a private entry, you're not likely to give your full name, address, job, list the names of all your relatives, etc.) Hence, if somebody else were to read that private entry, they might be rather perplexed and think, "Who is this person they're writing about? Why do they care about them? What's this place and why are they going there? Why are they doing that?" etc.

Perhaps if you wrote the diary entries as if they're private and nobody is ever going to see them, they wouldn't appear too redundant because they're just hints of what's going to be explained further in the story itself?

Yes, in this case it would be.

Blue -

You shouldn't be afraid to try anything. What's the worse thing that will happen if it doesn't come out good?

On the other hand, you'll gain some experience and just maybe come up with something really interesting.

Good luck.