Check for duplicate entry in a userform and fill in text boxes with duplicate information

Hi all,
I have created a userform in Excel 2010 for use as an inventory count entry form. After entering a tag number in txt1, txt2 - txt4 is automatically populatd. txt5 - txt6 are manual entries.
My problem is that a tag number should be entered only once. If the user enters the same number, I need to caution them that the number has already been entered and the entry needs to populate all text boxes with the information previously entered. If the quantity in txt5 then needs to be revised, the user is able to do so.