Forms :: Duplicate Multiple Text Boxes And Option

I am trying to run a check that will make the user aware if the request they are adding via the form is likely to be a duplicate. If so I want the option to allow or reject the entry. Alternatively I have seen solutions that just notify on the form is something might be a duplicate, by highlighting a textbox red for example.There will be duplicates across these fields but there are many other options included on the form, as I said this is just an indicataion.

The table is called
"MainAc"

The form is called
"SDC Request Form"

The comparison fields are called:
"First_name" - Text
"Surname" - Text
"Change_Number" - Number
"Date_from" - Date
"Date_to" - Date

Ive been trying the Dcount() function, IIF() function but no joy.I tried to putting the below in the before update event, but it didnt run at all, now Ive put it on the button click acion that duplicates records for me, however it just returns a non duplicate responce regardless.

Dim PreviousRecordID As Long
PreviousRecordID = 0
PreviousRecordID = DLookup("first_name", "MainAc", "first_name<>" & First_Name & _
" AND surname=" & Surname & " AND change_number=" & Change_Number)

[code]...

This was a copy and modify from an example database but I dont entirely understand what each part does.