Can I create a unique item in a SharePoint list to prevent duplication of entries, without deploying any app to the server?
Also if it can be done through SharePoint Designer, how it can be done??
Well good news. SharePoint 2010 seems to have this feature available.
Codeplex has project that has this feature along with others.
But if you want to do it yourself, you could possibly try this link? It seems to use an event handler to check if the item exists. Seems simple enough, although I have not tried it but I am curious if you get it to work.