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

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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.