if a user enter a value in a field, can we check that this value already exists in database and if yes then msg that this value already exists and empty the field for new entry and ask wether you want to see it and if yes takes you to the previous record. thnaks

Mabey this answer can help you in the right direction.
http://www.access-programmers.co.uk/ubb/Forum7/HTML/001935.html