Hi - I'm new to LabKey - it's a great tool, but I've come across a problem I hope someone can help me with.

I'm trying to create an example ETL that sucks data out of an external PostgresQL table and copies it into a list.
My ETL xml file is defined as follows.

<code>
<?xml version="1.0" encoding="UTF-8"?>
<etl xmlns="http://labkey.org/etl/xml">
    <name>Example: Cities to Labkey List</name>
    <transforms>
        <transform id="transfer1">
            <description>description</description>
            <source schemaName="cities" queryName="city"/>
            <destination schemaName="lists" queryName="MyList" targetOption="merge"/>
        </transform>
    </transforms>
    <incrementalFilter className="ModifiedSinceFilterStrategy" timestampColumnName="updated" />
    <schedule><poll interval="2m" /></schedule>
</etl>
</code>

This works for inserting newly added data. But when I modify a pre-existing row, the ETL fails with the following error:

ERROR: duplicate key value violates unique constraint "c2d22_mylist_pk"
...
Detail: Key (id)=(30) already exists.
Where: SQL statement "INSERT INTO list.c2d22_mylist (...)

It looks as though the filter is trying to INSERT on a duplicate key, rather than UPDATE as intended.
Am I doing something wrong, or have I misunderstood the documentation?

Many thanks.