Configure: warn about duplicate GENERATE declarations in build.info files

This sort of duplication is permitted, as the end result will be a single
item anyway, but we might as well warn to avoid future confusion.

Reviewed-by: Tomas Mraz <[email protected]>
(Merged from openssl#13824)

Author: levitte

Configure

@@ -2325,7 +2325,7 @@ EOF
             if ($generate{$gen}) {
                 $generator[0] = cleanfile($buildd, $gen, $blddir);
             }
-            $check_generate{$ddest}->{$generator[0]} = 1;
+            $check_generate{$ddest}->{$generator[0]}++;
 
             $unified_info{generate}->{$ddest} = [ @generator ];
         }
@@ -2439,6 +2439,9 @@ EOF
             warn "$_ is GENERATEd by more than one generator ($generators_txt)\n";
             $ambiguous_generation++;
         }
+        if ($check_generate{$_}->{$generators[0]} > 1) {
+            warn "INFO: $_ has more than one GENERATE declaration (same generator)\n"
+        }
     }
     die "There are ambiguous source file generations\n"
         if $ambiguous_generation > 0;