I know... Doing something dumb!
!insertmacro MUI_INSTALLOPTIONS_READ $R0 "..\email.ini" "Field 9" "State"
MessageBox MB_ICONEXCLAMATION|MB_OK "email is $EmailData Confirm email is: $R0"
StrCmp $EmailData $R0 Same NotSame
NotSame:
MessageBox MB_ICONEXCLAMATION|MB_OK "Your e-mail addresses do not match. Please re-enter."
!insertmacro MUI_INSTALLOPTIONS_WRITE "..\email.ini" "Field 1" "State" "Your name here."
!insertmacro MUI_INSTALLOPTIONS_WRITE "..\email.ini" "Field 2" "State" "Your email here."
!insertmacro MUI_INSTALLOPTIONS_WRITE "..\email.ini" "Field 9" "State" "Confirm email here."
(Text in the example MUI_INSTALLOPTIONS_WRITE is for example only, final version will write an empty string to the Field 2 one only)