After i created a duplication along a curve with this method I want to mirror the whole thing. If I add a mirror modifier on the curve and the object it doesn't the object is not transformed along the curve.

After i created a duplication along a curve with this method I want to mirror the whole thing. If I add a mirror modifier on the curve and the object it doesn't the object is not transformed along the curve.

Instead of using the duplicate method post din your questions you can use an array modifier and a curve modifier.
1. Add Curve
Add -> Curve -> Bezier
move it (in my case I moved it along the positive x-axis)
Place the origin at the start of the curve path

2.
Add a plane at the same location
Scale it down
Apply Location: Ctrl A Location

3.
Select plane, add array modifier Fit Type: Fit Curve Relative Offset modify to move in the direction of the curve

4.
Add Curve modifier to plane

5.
Add mirror modifier

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