A router hands out IP addressess using DHCP (Dynamic Host Control Protocol) and this should give a spare address to each device as it starts up and asks for one.
A router can be configured to use Static IP addresses for devices, you will know if you have done this, it involves accessing the router and each device and setting Static IP's.
What is happening in your case is either the DHCP on the router only has one address available to give out - Very unlikely - or the Tablet is set to use a static IP address which happens to be the first one available and has already been given to the PC.
Quick test. Shut both devices down. Restart the router, wait until all lights go green. Start Tablet and connect to internet Start PC and connect
If there is still a conflict - check router settings on router.
Check settings on Tablet - look under wi fi and look for a setting such as "Use this ip address" or similar