how to verify duplicate entry in textboxes?

I have various ul in which I have various li. The each li contain a textbox. Now the user has the possibility to add a new textbox to the li and fill in information.

My question is as 开发者_StackOverflowfollows: How can I validate that the new entry made isn`t same as any previous entry in any of the textbox in a particular ul? After the user has left the textbox, I must be able to pop any message if the entry is same as any previous entry in the ul

on onblur event of textbox you can write javascript to test if the value in the textbox is duplicate and then display your popup accordingly.