(edit - just realized this is a duplicate of
- sorry about that)
When a dupe has their shower interrupted (such as going to bed, having to use the toilet, or being ordered out by the player), the building itself remembers how far along the shower was. The orange progress bar remains, allowing another dupe to enter the shower and "use" the progress from the previous dupe's shower.
A player could potentially "prime" showers with a less valuable dupe, then bring a more valuable dupe (such as a researcher) in to finish the shower, allowing the more valuable dupe to get back to work faster.
Have a dupe interrupt their shower early before the orange progress bar completes, such as by ordering them to move to another location. Any other dupe (or the original one) can go into the shower and will continue filling the bar from where it left off.