Let's suppose that in two differents subcription we have 1 vnet with the same ip range, and both sites also have one subnet with the same ip range. Does VPN gateway still working ? For example in this case:
Subscription 1:
Vnet range : 10.1.0.0/16
Subnet range : 10.1.0.0/24
Subscription 2:
Vnet range : 10.1.0.0/16
Subnet range : 10.1.0.0/24
Both Azure networks use the same internal IP addresses, Vnet and Subnet ranges overlap, it is not possible to build a tunnel between these two Subnets as in both Subnets you may have PCs with same IP addresses assigned by DHCP.
VPN Gateway in Subscription 1 won't be able to determine where to send traffic, to PC in Subscription 1 or PC in Subscription 2.
However there is an option to configure a VPN tunnel between two Subnets with an advanced configuration using NAT and 3rd party VPN devices like Juniper, Cisco, etc.
For bidirectional VPN traffic between two end entities with overlapping addresses, the security devices at both ends of the tunnel must apply Source Network Address Translation (NAT-src) and Destination Network Address Translation (NAT-dst) to the VPN traffic passing between them.
https://www.juniper.net/documentation/en_US/release-independent/nce/topics/concept/lan2lan-vpn-jseries-srx-series-overview.html