Alert User of Duplicate Entry

I am looking to add to the begining the VBA below. The goal is for the code to first check if the data in cell A4 (on "Tracker" tab) is listed anywhere in column A on the "Store" tab and if there is a match, provide the user with a message that says "Already Entered" and stop the macro. If there is not a match, proceed... Help!

Sub NameUpdate()

Application.ScreenUpdating = False

Range("A4").Select
Selection.Copy
Sheets("Store").Select
Range("A2").End(xlDown).Offset(1, 0).Value = Range("A1")

Sheets("Tracker").Select
Range("A8").Select
Selection.Copy
Sheets("Store").Select
Range("B2").End(xlDown).Offset(1, 0).Value = Range("B1")

Sheets("Tracker").Select
Range("AL1:AL2").Select
Selection.Copy
Range("A4:A5").Select
ActiveSheet.Paste

Sheets("Tracker").Select
Range("AL1:AL2").Select
Selection.Copy
Range("A8:A9").Select
ActiveSheet.Paste

Range("H1").Select
ActiveCell.FormulaR1C1 = "1"
Range("W3").Select
ActiveCell.FormulaR1C1 = "1"
Range("A1").Select

ActiveWorkbook.Save
    
Application.ScreenUpdating = True
End Sub