Wrong incrementation of counter?

Question

Context
I would like to add automatically numbered highlighted comments in text.
Attempt
Following this answer I wrote the following
documentclass{article}
usepackage{color}
usepackage{soul}

newcounter{mycounter}
newcommandshowmycounter{stepcounter{mycounter}themycounter}

definecolor{aquamarine}{rgb}{0.5, 1.0, 0.83}
newcommand{my}[1]{sethlcolor{aquamarine} 
   protecthl{Comment showmycounter: #1} sethlcolor{yellow}}

begin{document}
some text 
my{my first comment}
some more text
my{my second comment}

end{document}

which unfortunately for some strange reason increments the counter in steps of five.

Question

Could you please tell me how to get a unit increment on this counter?

PS: I am a bit of a newbie at latex programming. Let me apologise in advance if this question is dumb.

Skillmon · Accepted Answer

Providing my comment as an answer: You shouldn't put stepcounter inside of the argument of hl. Instead increment the counter before it and only put themycounter inside hl:
documentclass{article}
usepackage{color}
usepackage{soul}

newcounter{mycounter}

definecolor{aquamarine}{rgb}{0.5, 1.0, 0.83}
DeclareRobustCommand{my}[1]
  {%
    sethlcolor{aquamarine}stepcounter{mycounter}%
    protecthl{Comment themycounter: #1} sethlcolor{yellow}%
  }

begin{document}
some text 
my{my first comment}
some more text
my{my second comment}

end{document}

egreg · Answer

The processing of hl requires multiple passes over its argument in order to do measurements. You discovered that there are five passes and each one increments the counter. Note that stepcounter acts globally.
You can avoid plunging into the innards of soul with some more work.
documentclass{article}
usepackage{color}
usepackage{soul}

definecolor{aquamarine}{rgb}{0.5, 1.0, 0.83}

newififstep
newcommand{stepcounteronce}[1]{%
  ifstep
    globalstepfalse
    stepcounter{#1}%
  fi
}

newcounter{mycounter}
newcommandshowmycounter{stepcounteronce{mycounter}themycounter}
newcommand{my}[1]{{% an additional group to do sethlcolor locally
  globalsteptrue
  sethlcolor{aquamarine}%
  hl{Comment showmycounter: #1}%
}}

begin{document}

some text 
my{my first comment}

some more text
my{my second comment}

This is again hl{yellow}

end{document}

By adding globalsteptrue you start the machinery that allows stepcounteronce to do stepcounter only the first time.
Note the additional group, which allows to avoid explicitly redeclaring sethlcolor.

Wrong incrementation of counter?

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