Why Mac Terminal remembers two consecutive commands when they are same?
You need to add the HISTCONTROL environment variable to your .bash_profile.  In your .bash_profile add the following line:

export HISTCONTROL=ignoreboth:erasedups


Close and restart your bash session and the dupes should be gone.  Alternatively, you could just execute that same line and it will take effect for that session (use it to test it out).



From the manual page for bash(in Terminal):


  HISTCONTROL
  A colon-separated list of values controlling how commands are saved on the history list. If the list of values includes ignorespace, lines which begin with a space character are not saved in the history list. A value of ignoredups causes lines matching the previous history entry to not be saved. A value of ignoreboth is shorthand for ignorespace and ignoredups. A value of erasedups causes all previous lines matching the current line to be removed from the history list before that line is saved. Any value not in the above list is ignored. If HISTCONTROL is unset, or does not include a valid value, all lines read by the shell parser are saved on the history list, subject to the value of HISTIGNORE. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of HISTCONTROL.