Detect Duplicate Record Before Entering

I have a subform that collects the following data to put in a record in tblClass:

StudentID, Trimester, SubcatID, WorkGrade, SkillGrade

Before the record is written for the first time OR edited/updated to the table, I want to search tblClass and determine if the new/updated info will create a duplicate record.

In this specific case, a duplicate record will be defiened by a record where the only fields being considered would be StudentID, Trimester and SubcatID. The fields WorkGrade and SkillGrade should not be considered.

The code I came up with was the following and it was put in the BeforeUpdate:

Dim conn As ADODB.Connection 'Connection Object
Dim rst As ADODB.Recordset 'Recordset Object
Dim strSQL As String 'SQL statement for open statement

' Create object variables
Set conn = CurrentProject.Connection
Set rst = New ADODB.Recordset

' Create sql to search for records in tblClass that match
'studentID, Trimester, and Subcatagory in the form record being added
strSQL = "SELECT * " & _
"FROM tblClass" & _
"WHERE fldStudentID = " & StudentID & " AND " & _
"fldTrimester = '" & Trimester & "' AND " & _
"fldSubcatID = " & SubCatID & ";"

' Open recordset
rst.Open strSQL, conn, adOpenKeyset, adLockOptimistic

If rst.RecordCount >= 1 Then
' record already exists in tblClass
msgbox "Record already exists!",, "Duplicate Record Error"
Me.cboSubcatID.SetFocus
Cancel = True
End If

' Close and disassociate object variables
rst.Close
conn.Close
Set conn = Nothing
Set rst = Nothing


This code worked great except when I went to edit an existing record. When I went to change a grade (WorkGrade or SkillGrade) on an existing record, it told me that I could not enter the record because the record already existed (ie, the record I had open and was editing). I am not sure if it is my code that I need to edit or if it is the placement of the code I need to change.

Any suggestions would be great.