You cannot define two modules that have the same name and different parameters. The second definition will overwrite the first. Or if you use the LOAD statement, the module that is stored will overwrite the current definition.
You can use define a module to have optional arguments that take on default values when they are not specified. For example,
/* return 1 if x[i] is within eps of y[i] for all i */
start IsEqual(x, y, eps=1e-6);
return( all( abs(x-y)<=eps) );
finish;
You can call the module as
b = IsEqual(x, y); /* Default: compare within 1e-6 */
or you can specify the third argument as follows:
b2 = IsEqual(x, y, 1e-12); /* compare within 1e-12 */
For more information on default and optional arguments in SAS/IML modules, see the section "Optional parameters and default values" in the article "Everything you wanted to know about writing SAS/IML modules."