Can I duplicate an in-app message instead of building it again?

Yes. Clone the campaign from the list:

Go to Campaigns → Instant in-apps.
Find the campaign. Use Search by name or code... or the status filters if the list is long.
Click the three-dot menu (⋮) on its row and select Clone campaign.
The copy opens in the campaign wizard with the same creative, trigger event, audience, and display settings. Change what you need.
Launch the copy, or save it as a draft.

The actions available in the menu depend on the campaign's status.