Finding "older" duplicates

[Trey]

I am trying to make a database for tracking calls, the caller gets one free
call (per 12 months), then is charged a flat fee for every call after the
first call in a 12 month time span. The data is imported into the database
weekly (about 120 calls a week) from a text file. I am able to query the
12-month part. But I am having trouble making a statement/macro/query to
check for duplicate entries with the same address in the database, with a
date and time older then the current record that is being processed to see
if an older duplicate exists. Then, if a duplicate exists, then the
"billable" field will be checked "yes", if a duplicate is not found then the
"warning" field is checked "yes".

Since someone from the same address may call twice in one day, they would
get the first call free, then they would be charged for next call. But I do
not know how to make the statement tell the difference between the first
entry from the following entries.

I am currently using Access 2000 for this project.

Any help would be very much appreciated!