This can happen when the report itself belongs to a particular security category which the system will interpret differently given the current infrastructure of the Report Writer tool.

The reason you are returning duplicate rows is because those constituents belong to multiple centers, one of which is the particular center that you are reporting for. The system exports this out as duplicate rows since this report is set up for center membership and it interprets that you want to know anyone who belongs to that center and their memberships, which will be returned as duplicates if they have membership in multiple centers.

There are 2 workarounds:

First, you can recreate the same report in the "General" security category (that is, to create the report from "All Centers" mode), then add a Report Filter for the security category in addition to the group filter. This will return the expected amount of rows.

Secondly, you can insert to the column "Center Name" into your report as is. Then, you can sort the column to view all of the constituents who belong to the center in question and remove the rest of the rows from the file.