With Gthumb it's possible to select multiple images using ALT or CTRL as a
modifier.
However, when using CTRL + leftmousebutton to "add" an image to the current
selection too quickly Gthumb acts as if the user made a double click on the
last picture.
Ex:
1. Select an image with left click
2. Quickly select a second image CTRL + left Click
3. If the second step is done fast enough Gthumb will switch to "single
image" display mode.