How to avoid duplicated records, when importing leads from excel?

Hi Guys,

I am new in Zoho so I am testing everything before start using the tool at full capacity. I create an excel with a few examples of leads with different fields and their respective information. When I import the file all is good at fist but then, when I import the exact same file it display all the leads info again (duplicate records) I try the three options (Clone, Skip, overwrite) but non of those are working! I am using the professional trial, should that be the problem? if not, How can I avoid to have duplicate records when importing different files?

Thanks,
Rolando