The question was: how to do something when a procedure parameter name is EQUAL to a column name.

Therefore, (parameter) vs. (column)
"user_name" <> "username" or
"user_name" <> "param_user_name" or
"user_name" <> "p_user_name" or ... - no problem.

But, if "user_name" = "user_name", that's the problem.