except it clones the mirrored object instead of actually mirroring your edits
not what I need
If using using the mirror modifier clones your mesh instead of mirroring it, it means the mesh is too far from the mirror axis.
The axis of each object is defined by the object centre. In the older Blender it was represented by large pink dot, it is probably orange in the new one.
Here's how to fix it:
1. Cut your mesh in half
2. Delete one half
3. Turn on the mirror modifier
4. Move your mesh closer to the axis
If your object ends up being off-centre, move it back in object mode to the 0 0 0 position.