Search 7.2.1: Duplicate results

System

We have migrated a content store from our current 6.7.2 Solaris environment to our new production 7.2.1 Linux environment. Search was installed on a separate physical server from the TeamSite server. After configuring the search/index server and building the index we are seeing strange results from some of our queries.

Query results are showing duplicate file entries in the result set when we are searching based on a custom date EA. The number of duplicates in the result set depends on the date that was used for search criteria.

Background:
We have an extended attribute defined for ExpirationDate on our files. We have a custom script that runs, executes a search using CSSDK to find expiring documents... then does some processing to expire the content.

Example of problem
A file has the ExpirationDate EA set to 05/01/2001 11:00. the file has 5 versions. When searching for the file by ExpirationDate, we get multiple duplicates depending on the date used to search:

On or before 6/1/2011 shows version 5 in the results 5 times
On or before 6/1/2001 shows version 5 in the results 1 time
On or before 12/31/2010 shows version 5 in the results 3 times

We get the same results in the Teamsite UI as we do using the iwsrchquery CLT.

We never saw these duplicates in our search results in 6.x. And we're not seeing the issue in all 7.x environments - this problem seems isolated to our production instance. We have wiped out and rebuilt the index, and still see the same results.

What could be causing this behavior?
Could it be related to # of Editions in the branch?
Has anyone else seen this type of behavior from Search?