When saving subnet, we should inform the user that it is not good idea to overlap these. From our manual:

While it is possible to define the same DHCP range in Foreman, it’s usually good practice to select a range from outside the pool defined in the installer, but still in the subnet. For the example above, it is recommended to define the DHCP range from 10.0.0.1 to 10.0.0.99 in the Foreman UI which gives the following IP address distribution:

- 10.0.0.1 - 10.0.0.99 - addresses reserved during bare-metal provisioning by Foreman
- 10.0.0.100 - 10.0.200 - addresses for dynamic clients in the subnet (discovered hosts, unmanaged hosts)

https://theforeman.org/manuals/1.15/index.html#3.2.3InstallationScenarios

There are some challenges (multiple ranges on one subnet) and #14396 needs to be fixed first.