I have multiple characters who do things... over and over again, in different timelines. But I have no idea how to reveal this to the reader. Any suggestions?
Your question could well be about the very people reading this question, Tomislav. :)
To explain - as of 12.53hr 18/05/2018 this question has been viewed 25 times, which equates to (assuming StackExchange's algorithm does not count the same person twice, thrice or quice(?)) your multiple characters.
If the other people viewing your question are anything like me then they will be running this question through their mind again and again in an attempt to discover its essence, which equates to your 'over and over again'.
And multiple timelines? Well, each of us might well be in a different time zone, but I don't really think that's what you mean. And that's the real question (for me) here - what do you mean by 'multiple timelines'?
Anyway, all else is standard.
Multiple people are revealed to readers by giving them different names. Try to make the names as unique as possible, so avoid Jane, James and John as your three characters (the reader will get confused). Instead go for something like Petra, Symon (yep, Symon) and Alexis. Actually, that makes me think about your different timelines. Standard names have changed over time - so Peter was Petros (in Greek times) and then Piers (in Norman times) and has now settled on Peter. Perhaps in the future it will be Peteg (when the Mongol hordes sweep over the world again).
Over and over again is handled by describing your scenery, people, dialogue and action in a sequential (not parallel because no-one's going to want to read that (unless you're going for a 'Choose Your Own Adventure' style book)) fashion. It's just like iterating over an array - there's always going to be something different in each iteration. Obviously there will be a variable to retrieve the records - this could be shown as a title to your chapter or section. And then, when the information is brought back, it can be described uniquely for each iteration because, let's face it - if not, you're going to have a very boring book on your hands.
Different timelines. Hmm. Not sure what you mean, but if it's about one iteration being in the 14th Century, the next in the 16th and the third in the 21st then why not just use labels? Each timeline/chapter could be prefaced by a date/place. Or you could include the labels in your narrative (Rome, anno Domini nostri Jesu Christi 371 and Petra was bloody ravenous). But if you're talking about parallel dimension then that's a little more tricksy. But still, labels will still do it for you - that's what we invented them for after all. If you still want something more fancy then why not look into denotational semantics of concurrency? Will Clinger has done some interesting stuff (technical term) in this domain.
Good luck with your work, Tomislav.