it depends on whether or not you specify the original recipient as a new recipient or not. This will forward a copy to an external address and keep a local copy:
address-to-forward@mydomain address-to-forward@mydomain, external-address@other-domain
Even though this seems to create a loop it doesn't as Postfix interprets this just as you want it to: "keep and forward".
The following won't keep a local copy, though:
Note that there are multiple ways to achieve "keep and forward". One often suggested is to use "recipient_bcc_maps". It works just fine but requires some more effort to set up properly on a UCS system as the "main.cf" is generated from templates, and those templates don't provide "recipient_bcc_maps" yet. We'd have to create our own template snippet, register it with the Univention config registry and re-build the "main.cf" file. Definitely not a lot of work, but still more than simply using the already provided "virtual" way.