| 4 Duplicate current layer (Layers > Duplicate). Flip your object (Image > Flip) or simply press CTRL+I.
Pick the Selection Tool , then press CTRL+A. Click your mouse once anywhere on the image, notice that'll create a selection on the flipped object. Now, drag the flipped image just below the original object (see below).
Deselect selection when done (CTRL+D).

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