Hi Carol,
Where you are recording a previously reported/recorded county or national rarity please note the following BTO-generated comment which hopefully answers your general query:
"Many thanks for using BirdTrack and for putting in some of your historical records. When adding old records, there is no need to complete any local rarity forms that are presented, unless the bird was not submitted at the time / does not appear in the relevant local bird report. Please see the following link for further information:
http://www.bto.org/volunteer-surveys/birdtrack/bird-recording/rare-scarce-birds
For the xxxxxxxx and other 'known' local rarities, please just circumvent the form by pressing the 'BirdTrack Home' button underneath it. This will prevent an email being generated to BirdTrack and the local bird recorders."
I hope that make sense. If you follow the imbedded link it explains how potential duplicates are dealt with. Apart from that I wouldn't worry about it in any case as it is surely better to submit a duplicate record (that can be simple ignored/deleted by the recorder in a matter of seconds) than to think your record hasn't been made available at all.
Hope that helps.
Chris A.