If you define them both yes, but you're still free to use them both.

  var my_var = 1
  myVar += 1
  echo(my_var)

Will happily compile and give you 2. I'd prefer an error in that case because while I'm not liable to make that mistake but I could easily mistype long_variable_name as long_variablename without noticing and then cause myself problems when I try to grep.