It is not unusual to see a copyist at work in many museums. This has been a way artists learned from the masters for many generations.
Each museum has its own rules for allowing a copyist to set up an easel and work, the most general rule being that no copies can be made that are the same dimensions as the original work of art,
those dimensions of course being documented in great detail in records of the original painting.
For obvious reasons, this keeps copies of famous art works from being palmed off on the unsuspecting buyer as the original work of the artist being copied.