Yes it is possible for multiple devices to have the same hostname on a network, you can simply change the name of any two computers on a network and they will share the same hostname. Unfortunately this can cause a number of different problems, like domain name resolution will not work correctly since they are both sharing the same hostname. Though they can get different IP addresses assigned via DHCP since that uses MAC addressing to assign IP addresses.
MAC addresses are group assigned to manufacturers much like IP addresses are assigned to ISPs, so that's not unlikely if both devices are from the same manufacturer.
Probably not. If you have any friends or family or co-workers (small-business or home network), that have a similar android device and have accessed your network, that entry may be listed in your DHCP Leases/Connected Devices list. If you delete the DHCP lease/disconnect the device in question and it pops up again, it could be caused by a number of things: some devices (phones especially) have multiple MAC addresses and can have multiple DHCP entries, someone could have left a device there, it could be a cracked or stolen wpa-psk key (though I doubt it), and a device spoof seems really unlikely.
Hope that helps!