[rantwasp]

you cannot peer 2 vpcs that have an overlap but you can have multiple vpcs that have overlaps. it only matters for the 2 vpcs you want to peer

[vorpalhex]

But AWS tells you to not overlap them, and likely keeps that behavior for legacy reasons.

[moduspol]

I could have missed it, but I've never seen a suggestion not to overlap network addresses unless you want to peer them.

If you're launching ephemeral networks for testing VMs / virtual appliances in their own, isolated networks, it can be totally feasible to have lots of them using the same addresses. You can only create 5 VPCs (at all) by default per AWS account, but they'll raise that limit for you if you request it.