Detect several cases of comparing something with itself and then warn the user

Detect several cases of comparing something with itself and then warn the user. That avoids some occasionally harmful errors, for example: when the user thinks that two files are equal... but it's not true (because one file was compared with itself).

The code of the present request has been tested and also used habitually, no problems were seen but I'm not a Kompare expert and I wanted to ask for other points of view.