Is it unusual for people to be in the middle of writing one novel and a second idea comes to mind? I have been writing my book for about a month and in 11,000 words and 7 chapters, 23 pages in, and I already have an idea for a second book. Not the same characters, but a second book? I will finish the first before I start the second, but I have been putting my ideas down on notes, but I do think its strange that I have 2 books rolling around in my head at the same time.
Oh I've got like a dozen different book ideas that need to be written. Some need more polishing than others, but the seeds are there. You've got a good idea making notes for what comes to mind, I do something similar. Keep them somewhere you can find them later. I find it helps me to be able to set it aside and concentrate on the story I'm currently working on to know the other ideas are stored somewhere other than my own head.
I often have ideas for the next book I'm going to write while writing another one. Since I personally can't work on more than one story at a time, I just type up notes in word so I don't forget any of the details.
I currently have rough-to-detailed plans for 2nd-4th installments of the Doctor Who fan-series I started 1st-4th installments of a Frozen fan-series I have not yet started a short story that's been in the back of my mind for maybe a year now and about half a dozen sequels to the Urban Fantasy WIP I'm working on
Nah, it's perfectly normal. I've got three novel ideas floating around, and maybe half a dozen short stories and flash pieces.
I've got three novels that I've started writing. I rotate around until one hooks me, then concentrate on it. If I have another idea, I will scribble down all ideas. Get cheap notebooks, and use a different one for each book.