This is a form with a subform The main form displays rows from the "People" table. The subform displays rows from the Class Registration table, linked by the Email field, which is the primary key for the People table, and part of the key for the Registration table.

The subform contains a control cmdAddRecord1, which copies data from the main form and inserts a row into the Registration table. The VBA code runs OK, but when I use the navigation button to move the main form to another record, it attempts to write a second row to the Registration table, and I get the error message about a duplicate record. With he primary key removed, it actually writes two identical rows.

Here's the VBA for the subform: BTW, cmdDelete works OK
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Option Compare Database
Option Explicit


Private Sub cmdAddRecord1_Click()
On Error GoTo Err_cmdAddRecord1_Click
    Dim lngRecordsFound As Long
    Dim txtAddSQL As String ' Define INSERT Statement
    txtAddSQL = "INSERT INTO Registration (ShortName, ClassNumber, Email, First, Last) " & _
        "VALUES (txtShortNameS, txtClassNumberS, txtEmailS, txtFirstS, txtLastS);"

    Dim txtFindSQL As String ' Define SELECT Statement
    txtFindSQL = "SELECT * FROM Registration WHERE ShortName = txtShortNameS AND " & _
    "ClassNumber = txtClassNumberS AND Email = txtEmailS;"
    '--------------
    txtFirstS = Forms![Registration Update Form].txtFirst 'move person data from main form
    txtLastS = Forms![Registration Update Form].txtLast
    txtEmailS = Forms![Registration Update Form].txtEmail
    txtShortNameS = Forms![Registration Update Form].txtShortName
    txtClassNumberS = Forms![Registration Update Form].txtClassNumber
    txtGo = MsgBox("OK to Update?", 1, "Is All Data Correct")

    If txtGo = vbOK Then
        DoCmd.RunSQL txtAddSQL
    Else
        txtAction = "Cancel"
        Me.Undo
    End If

Exit_cmdAddRecord1_Click:
    Exit Sub

Err_cmdAddRecord1_Click:
    MsgBox Err.Description
    txtAction = "Cancel"
    Me.Undo
    Resume Exit_cmdAddRecord1_Click

End Sub

Private Sub cmdDelete_Click()
' Delete the Registration Record
' This will delete both records if there are duplicate keys
On Error GoTo Err_cmdDelete_Click

    Dim txtDeleteSQL As String
    txtDeleteSQL = "DELETE FROM Registration " & _
    "WHERE ShortName = txtShortNameS AND ClassNumber = txtClassNumberS " & _
    "AND Email = txtEmailS;"
    '-------------------------
    DoCmd.RunSQL txtDeleteSQL

Exit_cmdDelete_Click:
    Exit Sub

Err_cmdDelete_Click:
    MsgBox Err.Description
    Resume Exit_cmdDelete_Click

End Sub

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I had a similar problem a whistle ago
This was cased by the sub form being based on the main forms table e.g. create a record in the sub also creates it in the main form once the forms been re queried
I would start by checking sub forms data relationship to the main forms data
If they are from un related tables check the relationship in the relationship form perhaps remove it for now until resolved.
I’m no expert hope this helps if not there are more clever people out there best of luck