Modify code to show message box on duplicate entry

Hi everyone.. i am using the following code to enter data into a worksheet. I would like to make it so if the data being entered in textbox1 is already present in columnA of the worksheet then a message box will appear stating so, and none of the data from the form is entered onto the sheet.

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Here's some code that I wrote for ComboBoxes but you can adapt it easily

You'd better prevent that users enter data that are not being allowed.
To show a message after all the work has been done must be rather frustrating.
Use the principle: users can't make mistakes, programmers can forget to help users to do so.

If Column A contains unique entries then use a ComboBox instead of a TextBox

Amended the code to check with a TextBox

What's the problem using a combobox ?
The user can enter any text (s)he wants. As long as the indexnumber is -1 it's not a duplicate.
You can even use the combobox1_change event to change it's backcolor to indicate whether the text entered is a duplicate or not.

Alternative using validation; no comboboxes needed.

Cfr. the attachment