Prevent duplicate entry

Hi,

First of all I would like to thank everybody that support this site, I think it’s a great source of
Informations for a rookie like my self and many others. BRAVO !!!

Now down to business this is what I am trying to accomplish.

Sub CommandButton1_Click()

Worksheets("sheet1").Range("C10") = UserForm1.TextBox1.Value

If Sheets("sheet1").Range("C10") = Sheets("sheet1").Range("A:A") Then

UserForm2.Show

Unload Me
End If
Sheets("Sheet1").Select
Range("C10").Select
Selection.Copy
Range("A30").Select
ActiveSheet.Paste

Unload Me

End Sub

I want the value of “ C10”(part number) to look in column “A “ and see if the
Part numbers already exist.

This reference is my problem { If Sheets("sheet1").Range("C10") = Sheets("sheet1").Range("A:A") Then}
If I use a single cell as reference it works exactly how I want it to, but as soon as
I try to to use column “A” as a reference I get that familiar Debug box I think
I have seen that dam box 1000000 time.

So please any help would be greatly appreciated

Thank you,
Marc