Preventing Duplicate Entries

Hi

I have a userform where users enter data and select a date and a time

Worksheet ("CVdata")
Column A = Date
Column B = Time
Column C = Gen Debt

Userform
Date text box = CVdate
Time comboBox = ComboBox1

I want to add a code in my Add data button which will prevent users from entering data which has the same Date and Time as a record on my worksheet

Looked through the forums but cant seem to find an example of this, Please help?