Two is better than one! - A really easy task!

Here are two different examples.

Two example images, doubled in the next examples.

(Click on the image for the larger size image).

1/. Find an image which you want to double.

2/. In Photoshop - Image Size.

3/. File new - the same height, but double the width.

4/. Copy the image to one side of the new image.

5/. Then with the original image - Image Rotate - Flip Horizontally.

6/. Copy paste to join the original image.

7/. Use the Move Tool to align the images to the best join.

8/. Depending on the alignment you may also need to also crop the image.