There is an option to use any image file in a four ways: Original - use it as is. H flip - use horizontally flipped image. V flip - use vertically flipped image. 180° - use 180° rotated image.
Program will understand that flipped images are variants of the same image and will not place original
image and it's flips side by side.
To activate this feature you need to allow to use duplicates in the Advanced preferences.
After option was activated you will see a list of check boxes at lower left of the main screen.
To use only original (not flipped/rotated) images - check "Original" check box only.
To use originals and flipped horizontal images - check both "Original" and "H flip" check boxes.
To use originals and 180° rotated images - check both "Original" and "180" check boxes and so on...
When you run render with some flipped check box (other than "Original") for a first time, program will
add information about flipped images to the base. It may takes some time. This information added
permanently i.e. second render with same flips set will start immediately.