Is there any way to prevent Textbox from duplicate entries?

Is there any way to prevent Textbox from duplicate entries?

I am working in asp.net project.I want to prevent my textbox from repeated same entries.I use autocomplete and checkbox values as input for my textbox.Is there any way to prevent Textbox from duplicate entries from pasting,typing,etc..once i click on the existed values from autocomplete means it have to disappear from the textbox..Please help me out guys