No. macOS shows one Dock, on one screen at a time, and there is no setting or third-party app that changes it.
What you can do is choose which screen it lives on, and move it to another one on demand. Both are below.
The Dock is a single process drawing a single window. It is not duplicated per display the way the menu bar is, and Apple has never offered an option to mirror it. No preference, no hidden defaults key, no utility. If a page promises you a Dock on every monitor, it is describing the menu bar, or it is wrong.
This is one of the few Mac Dock questions with no workaround, so the answer is stated plainly.
Most people who search for this have a Dock that keeps moving between screens on its own, and they want it available wherever they are working.
The Dock follows your pointer. Push the cursor to the bottom edge of any display and hold it there for a moment, and the Dock relocates to that screen. It is deliberate behavior, though it can feel random or like a fault when it happens mid-task.
If the problem is a Dock that wanders between screens, pin it to one screen so it stops. The fix is covered in how to keep the Mac Dock on one screen.
Since you cannot have it everywhere, the next best thing is summoning it where you need it. Push the pointer to the bottom edge of the display you want, and keep pushing past the point where the cursor appears to stop. The trigger sits slightly beyond the visible edge, which is why it often seems not to work.
If the Dock is on a side edge, aim for that edge.
Open System Settings, then Displays, and click Arrange. Drag the white menu bar strip onto the display you want as your main one. The Dock treats that screen as home: it lives there by default and returns there after sleep or reconnecting a monitor.
Setting the main display decides where the Dock starts and where it comes back to. The Dock can still move.
The menu bar can appear on every display, and a lot of the confusion starts there. In System Settings, Desktop & Dock, under Mission Control, turning on Displays have separate Spaces gives each screen its own menu bar and its own full-screen spaces.
The Displays have separate Spaces setting does not do the same for the Dock. There is still one Dock, on whichever screen the pointer last summoned it to.
Want the Dock to stay on one screen? Still Dock, which locks the Mac Dock to one screen, pins it to the display you choose, without turning off separate Spaces and without moving it to an edge you did not pick.
No. This comes up often enough that it is worth being direct: no third-party app does it, because the Dock is a single system-owned window that apps cannot duplicate. Apps that manage the Dock, this one included, can choose which screen it sits on. None can change how many screens show it.
No. It gives each display its own menu bar and its own full-screen spaces. The Dock stays single and still follows your pointer.
Because you pushed the pointer to the bottom edge of that screen and held it there, usually while reaching for something. It is standard behavior on every Mac with more than one display. See how to keep the Dock on one screen.
Yes. macOS has no setting for this, but the built-in workarounds and their costs are laid out in the multi-monitor guide.
No. The number and kind of displays makes no difference. One Dock, on one screen, on every Mac.
Related: keep the Dock on one screen and what killall Dock does.