Send Row Number Of Changed Cell To Cell Where Row Resides In Cell

I have the following range of cells B2:Y3700. I want that when a value from 1 to 9999 is inserted in one cell of the above mentioned range, to copy the row number of that cell in the cell that is located in column Z and row is the number inserted in that cell. For example, if the number "1234" is inserted in one cell from range B2:Y3700, to enter in cell Z1234 the number of the row where is located the respective cell. At same the time is very important to me to not permit to insert the same number in B2:Y3700, so i believe that is need a macro code to check the cell from column z to see if value already exist . Sorry for me english, Thanks.

Re: Send Value To Specific Cell

Thanks for your reply Simon Lloyd. I try your code in a new Worksheet and i insert random values to see if is working, but after many try's i don't know what this code does. I need a code that if a number is inserted in a cell from a specified range to send the number of the row in a cell that is located in column Z (or any letter) and the row is the number inserted in that cell. It is very important to not permit entering of a number that have been already inserted in that range. The attached file has no formulas or macro code, it is manual completed for detailing my needs.
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Regards,
Marius

Re: Send Value To Specific Cell

Sorry Simon for my reply post, i paste the code in "ThisWorkbook", not in Sheet. The code work very good, but i need that after i delete or edit a value from C1:G20, the column A must be corrected. This way i will avoid the error "Run time-error 1004: method 'range' of object '_Worksheet' failed " .To denied duplicate values i modify the code :

Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Count > 1 Then Exit Sub
    If Not Intersect(Target, Range("C1:G20")) Is Nothing Then
    If Range("A" & Target.Value).Value > 0 Then
    MsgBox "Code already exist"
    Target.Value = ""
    Else
        Range("A" & Target.Value).Value = Target.Row
    End If
    End If
     
End Sub

Re: Send Row Number Of Changed Cell To Cell Where Row Resides In Cell

One more time i need help. My final macro code is

Private OldValue As Variant


Private Sub Worksheet_SelectionChange(ByVal Target As Range)
OldValue = Target.Value
End Sub


Private Sub Worksheet_Change(ByVal Target As Excel.Range)
    If Target.Count > 1 Then Exit Sub
    On Error Resume Next
        If Not Intersect(Target, Range("B1:AT3700")) Is Nothing Then
        Range("CX" & OldValue).Value = ""
    End If
    If Not Intersect(Target, Range("B1:AT3700")) Is Nothing Then
    If Range("CX" & Target.Value).Value < 1 Then
    If Not Intersect(Target, Range("B1:AT3700")) Is Nothing Then
        Range("CX" & Target.Value).Value = Target.Row
    End If
    Else
    Target.Value = "Cod Existent"
    End If
    End If
End Sub

My new code resolve the previous problem, with the edit or delete value from cell, but when i select multiple cells and press delete key to clear values from them, or i drag with mouse the value from a cell to other cells the macro code is skipped. Is possible to make the macro code to run like a formula in excel, or to not permit editing or deleting of more than one cell.
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Regards,
Marius