In the 60s and 70's the hard cover albums were thought too expensive and the comming out was similar to books: a good edition in hard cover and one less expensive in a soft cover. That lasted into the 80's. Now soft covers only exist in france and belgium for marketings coups.
So if I count correctly I have bought some of the stories 4 times: first in the comic paper Spirou (2 pages per week), then once as soft cover, then a hard cover edition (when I got a raise in pocket money), and now in the intégrals...