Traceroute hops in the same subnet

each router interface should be in different subnet?

The error in your logic lies in the assumption that network topologies always involve routers which strictly pass packets received on an interface in one subnetwork out via an interface defined in a different subnetwork. While that is true in common SOHO and medium-enterprise environments, it is not the only possible mode of deployment available. Network engineers might have reasons to adopt another approach, or through luck and configuration churn, they might cause one to be adopted accidentally.

There are several scenarios in which a router can receive packets which are subsequently routed back to the same network they were received on. It matters how and with reference to which device that we define our subnets. For example, it is entirely legal for a router to have an interface of 10.0.0.1/16, while devices sat behind it use addresses in the subnet 10.0.x.y/24. From the device's perspective, packets from 10.0.1.x to 10.0.2.x would need to traverse one or more routers, but the router knows better. This phenomenon of network tromboning is commonly found in the core of enterprise networks. While not necessarily efficient, it does work and the router is doing its job correctly.

Another common occurrence occurs when running a traceroute from a device connected to a VPN concentrator, which tunnels its traffic over the VPN to the remote device. If we suppose this was the cause of your traceroute output, the .14 device would likely be the concentrator connected to the corporate LAN at the remote end. The .15 device is the router for the network the concentrator is connected to. Packets are received (tunnelled) on the .14 interface, and passed back out the same interface to be routed onwards to their destination. There's nothing wrong with this, and traceroute is legitimately showing you the identities of all the devices responsible for touching the packets at the IP layer.

Without further precise knowledge of the network topology, and in particular the configuration of the intermediate router(s), it is impossible to understand precisely what is occurring in your case.