Delphi 1 - Error #70 - Please help

I am getting, in a more or less random manner, compile error #70
saying approximately "Wrong unit version unit_name" (I get the text in
French because I have a French version of Delphi) meaning that
"unit_name" has not been compiled with the right Delphi compiler
Actually, "unit_name" is a unit from another project of mine, which I
compiled with this very same Delphi! The search path includes the
directory where the unit_name.DCU file is located, but not the
directory where the unit_name.PAS resides.
I can bypass the problem by just recompiling the project to which
unit_name belongs, but this is rather inconvenient and also makes me
wonder about potential problems if I was to redistribute my code...
Could someone out there provide some assistance?
                                                Pierre Gemis