I'm ASSUMING this isn't on purpose, anyway. But basically if Duplicant A enters shower, then gets out when their bar is at 80%, that bar will remain. Then, if Duplicant B enters the shower after A gets out, B will start his shower from 80%... meaning he gets clean MUCH faster.
Basically, showering is being treated as a task to be completed by anyone like research or building, rather than being done on a person to person basis.
1) Have a duplicant enter shower and shower for a bit but not finish. 2) Use "move to" command on duplicant to make him leave shower. 3) Wait for another duplicant to use shower from where he left off.