titles must be unique

That is usually correct.
Import files usually do not include the node id - which would be needed to be sure of what data is replacing what node. If your import file file does include a nid field, there would probably be a chance of supporting replacement. I'm not sure if even that is available however.

Indexing on title is problematic (which may seem like a quick idea) as if the title changes in either end, they would no longer match up. And other cases may have more than one page named the same.

Anyway, try setting the nid - if possible

No update possibility yet. Also not possible to map the "nid".

I will implement the "title must be unique" again as a temporary "solution" until updating works. Expect it in -rc5.

Subscribing... Thanks for the ugraded version.

A suggestion: it would be nice to have a choice of which field to use as the key for updating e.g. apart title (as in version 5 suggested above), any of the cck fields.

For instance, if a specific CCK field was added for mainting the unique key, this should be made available. As an example, for Products, these fields will be one of the cols in the content_type_product table. The User then has a choice to decide what should be used in the update process.

A good candidate for a unique key in 'Products' would be uc_products.model, the SKU field.

Indeed, I have a CCK field called article number that has to be unique. This field also exits in my ERP software.
I want to export a csv file from this ERP program and import it into Drupal to update the prices.
So, it would be great if I could select the field that holds the unique key.

Yes, the SKU field is the one to index on.
We just need the import to be able to index on a foreign key - and merge/update when it finds it.

...I've done this for my import_html where I index on path and merge/update pretty well over top of existing nodes. Can be done.