You can always manually update the trigger in the generated package. It is a separate step - you can’t set this in Modeler directly.
You should not get duplicate process models by having concurrent trigger setting though - concurrent trigger just means that the IS will handle more than one document at a time, not generate duplicate instances. Using serial will mean that you can only start one process model at a time or be processing one step at a time. Using serial will only affect order and concurrency but it will not actually change the handling of the inbound documents.
In most cases, you will want to use Concurrent triggers.