Blatant copy-pasting is tedious and immersion-breaking.
All games require duplication and copying. An artist can’t be expected to create separate, unique asset for every tree and man-eating dog, and the engineer can’t possibly give all enemy mobs unique and particular personalities. So copying is required - but designers should always attempt to arrange duplicated elements in engaging and complex ways, helping to hide the duplication from the player. A designer should never simply scatter a featureless space with obviously copied enemies - it makes the duplication so obvious that immersion is broken and play becomes tedious.
Dark Souls 1 has some amazing level design and some terrible level design. The first major level of the game, the Undead Burg, uses a small set of enemies in a wide variety of challenging scenarios that never feels tired or stale. But in later levels, things fall apart - the most clear example being Lost Izalith. A major field containing a few important upgrades is guarded by a series of Taurus Demons. They stand almost shoulder to shoulder, on a flat featureless plain. The effect is that it’s obvious a level designer simply copied a single enemy out into a big group.
Because there is nothing to contextualize the crowd’s presence, and there’s nothing in the terrain to make the fight terribly challenging (as players can simply aggro individual demons and fight them away from the group), eliminating these monsters feels like a pointless chore. The encounter is devoid of rhythm and is a shocking contrast to the tight, engaging enemy layouts in the Undead Burg.
Final Fantasy XIV Edit
The original 1.0 release of Final Fantasy XIV was notorious for its copy and paste map design. It was very obvious that maps were constructed from a few different area props. Maps were very big, but empty and lifeless, and very boring to walk through. Since teleporting to different areas was very limited in the original 1.0 release, sometimes you had to walk places as well. Needless to say, FFXIV was very poorly received at its original release.