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Update with group_concat
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You have an update which runs a sub query which returns multiple rows, what are you expecting it to do? You have nothing in there which specifies which of the (potentially) multiple returned rows it should use in the update, so it uses the first one it gets back.

In other DMBS's like Postgres, a subquery which return multiple rows will raise an error. SQLite apparently allows it and just uses the first returned row.

If you're trying to update Plassnommers based off of the Aliasnaam field, it would be something like this I believe

update wingrd13
set Plaasnommers = (
    select group_concat(Plaasno)
    from wingrd13 as foo
    where foo.Aliasnaam = wingrd13.Alasnaam
);

Note that if the field you were updating were used anywhere in the sub-select to the same table, then it would probably result in "undefined" problems as it would be updating it and changing the results while you were going through it.