hello.
How would I connect 2 networks together with the same subnet in a IPsec tunnel?
your proposal is not standard and doesn't scale well in the future. so make a new scheme.
Apparently Mikrotik was smart enough to know it already had the subnet in it and would not let the IPsec tunnel start.
no. it wasn't all about the router. even your lan clients won't do routing for the same exact subnet.
My mind is telling netmapping but not sure where to start.
This would be my IPsec path
172.16.6.0/24 -> 172.16.6.0/24
if you want to try,
do split that 172 subnet into 2 parts.
gather all scattered ip on both sides, and move them to their new subnet.
if you can't, just make a new subnet.
and next time you draw a network, make sure you have hierarchical and scalable scheme.
do simple routing. and make your day easier.
good luck