This situation is called an IP conflict.

Data from a server is sent in small bits called packets. In an IP conflict, some packets will end up in one computer while some other packets will end up in another computer.

So, overall none of the computers get the data as the original message or picture being sent by the server cannot be retrieved with only half the packets. So it will show up as an error.

If two computers on the same network have the same IP address … well, the results are unpredictable. It depends on how all the other equipment on the network handles the problem. In general it could be as bad as both machines not being able to communicate at all. More likely only one will not be able to, or perhaps there will simply be intermittent problems on both.

Internet protocols work by sending small, individually addressed messages. Each message can be routed differently. Connecting to a website usually involves at least tens of these messages, and often thousands. If you only get half of the messages, while the other computer with your IP address gets the other half, nothing will work.