I have an animation done for one character but since I have more of them they need to use the same animation, can I copy the rig and give them the same animation or I have to do everything again from scratch?
Copy the rig in object mode (shift-d), the new rig will have the same name with a suffix of .001, you can rename the rig as needed.
when the rig is copied, only the rig’s name changes, all the bone’s names stay the same. You can assign any of the first rig’s actions to the dupilcated rig in the action editor, since only the rig’s name changes and not the bone’s names it will work.
In fact, I think when you copy a rig, the currently active action will be assigned to the new rig as well. But any action from the first rig will work on the 2nd rig.