Make sure the version of .NET Framework installed on the server is at least 4.7.2. If so, then this issue is caused by an identified product defect. There are two columns in the "dbo.ModificationSchedule" table which should have uniqueness constraints on them. When the "InitializeDatabase" SQL stored procedure is executed multiple times (usually manually when a user is manually setting up a database), it is creating multiple entries in this table which leads to the error. The defect ID for this is VSTS000253291 and will be included in the next version of GPOADmin.
WORKAROUND:
Request and analyze the data contained within the "dbo.ModificationSchedule" table in the GPOADmin database and make sure each container has exactly one row containing the XML description of the "ChangeWindow" data for that container. Locating and deleting all but one row for each container will correct the issue.