This was first reported in the letter of Maulvi Abdullah to Mirza Ghulam Ahmad written at the end of the 19th Century.
The footprints, as indicated in the picture below
by Prof. Dr. Fida Hassnain, clearly show that the
sculptor was depicting the feet of a man who had
been wounded, and he wished to portray this in some manner....
Clip from 'Did Jesus Die?' - Feet Carvings
This Video shows an amazing 3-D reconstruction of
what the feet must have looked like that are depictedin the footprints. The 3D model shows how the marks
could be explained, with a single nail going through
both feet with one on top of the other.
The idea of one foot being placed over the other during Crucifixion also matches with the figure in the Shroud of Turin. That person too had one foot over the other.
Source : http://www.tombofjesus.com/index.php/en/the-rozabal-tomb/feet-carvings