You are attempting to create a new element in your program
and have given it a name that is already being used.
 
For example:
  - A CONST statement uses the same name as an existing
    variable.
  - A new variable or procedure has the same name as an
    existing procedure.
 
Starting a variable name with the letters "FN" is another
way to generate this error, because FN is used exclusively
with DEF FN.