I'm trying to develop chat system in php, sql and ajax. I created function by ajax to get messages from database this function its event when window upload, so if i open 2 windows in browser to test the application, I found the messages bu when i send message it appear in just the window which send from not both of the 2 windows. To solve this problem i used setInterval function every 1 second to show messages. Do this huge requests damage the server ??
I don't quite know what you meant with "Damage", but nothing can be really damaged by a few extra requests.
If you're wondering whether the webserver can handle the load, it really depends on how many chat sessions are going at the same time. Any decent web server should be able to handle a lot more than two requests per second. If you have thousands of chat sessions open, or you have very CPU intensive code, then you may notice issues.
A bigger issue may be your network latency. If your network takes more than a second for a round-trip communication with the server, then you may end up with multiple requests coming from the same client at the same time.