I think I can see what's been done there.
Two cables come down the wall, They are probably the ring in and out. When the house was built they probably went to one socket.
Subsequently, the ring has been slightly enlarged, so it now goes into LHS socket, across to RHS socket, out of RHS socket. It is still a ring and both sockets are on a ring, neither is a spur. This is quite correct.
So you can take a spur off either of these sockets and put another double socket on the end of either or both of them. You can put a 13A load on any or all of those sockets (even though they are doubles).
If you want to have more than two new sockets, there is another easy and correct method to do it.
To verify that the cables coming down are the Ring In and Out, you can check for continuity between Red and Red, and between Black and Black, on those two cables.
p.s. if you just want to plug in your PC cluster, it is simpler to just use a multiway extension. You can get these with surge protection built in for a few pounds. PC current loads are very low.