GIMP - Symmetrical image, trying to duplicate layer and flip. Having trouble

I am making a symmetrical image. I have half of it done and I want to flip it so It joins up nice.

What I have read so far is duplicate layer then just flip horizontal. I have done that but it flips both layers, the original and the duplicate layer. Any way around this or whats happening?

Hi davep1234,
I don’t use Gimp, but I guess a duplicate-layer is always synchronized with the original.
So what happens if you make a new layer (instead of duplicating), and then copy/paste the image and flip it in the new layer?

Increase the canvas size to fit the finished image. Choose “layer to image size”.

Copy your image and paste (which positions it on top of the existing image), then flip which lines it up exactly on the other side of the canvas.