> My main problem is that asking for currentitemnumber, currentitem increases the index count in the 24 items set [...]
No, what makes you think that it does? Reading the currentitem or currentitemnumber property *does not* cause any selection or increase some index count. It passively returns the current state of the respective stimulus element.
One problem you have in your code is that you query currentitem / currentitemnumber /ontrialbegin. That's *before* any selection for the respective <picture> element has even occurred in the given trial instance. Selection won't happen until the stimulus is prepared for display, which happens *after* any /ontrialbegin logic. If you want your values to reflect what actually happened in the trial, you should move those statements to /ontrialend.
To illustrate the difference, run
<values>
/ currentitem_ontrialbegin = ""
/ currentitem_ontrialend = ""
</values>
<text mytext>
/ items = ("A", "B", "C", "D")
</text>
<trial mytrial>
/ ontrialbegin = [trial.mytrial.insertstimulusframe(text.mytext,1); values.currentitem_ontrialbegin=text.mytext.currentitem ;]
/ ontrialend = [values.currentitem_ontrialend=text.mytext.currentitem]
/ validresponse = (57)
</trial>
<block myblock>
/ trials = [1-4=mytrial]
</block>
<data>
/ columns = [trialnum trialcode stimulusitem values.currentitem_ontrialbegin values.currentitem_ontrialend]
/ separatefiles = true
</data>
and examine the data file.
You should also consider whether you have to call resetstimulusframes() /ontrialend. It's impossible to judge from just the code snippets whether that is necessary or not. In this regard, it is important to realize that any stimulus you add to the trial's stimulus presentation sequence via insertstimulusframe() etc. will persist across all instances of the trial if you don't do a reset.
Again, to illustrate compare
<text a>
/ items = ("A1", "A2")
/ position = (50%, 48%)
</text>
<text b>
/ items = ("B1", "B2")
/ position = (50%, 52%)
</text>
<list mylist>
/ items = ("a", "a", "b", "b")
</list>
<trial mytrial>
/ ontrialbegin = [if(list.mylist.nextvalue=="a") trial.mytrial.insertstimulusframe(text.a,1);]
/ ontrialbegin = [if(list.mylist.nextvalue=="b") trial.mytrial.insertstimulusframe(text.b,1);]
/ validresponse = (57)
</trial>
<block myblock>
/ trials = [1-4=mytrial]
</block>
<data>
/ columns = [trialnum trialcode stimulusitem stimulusitem]
/ separatefiles = true
</data>
against
<text a>
/ items = ("A1", "A2")
/ position = (50%, 48%)
</text>
<text b>
/ items = ("B1", "B2")
/ position = (50%, 52%)
</text>
<list mylist>
/ items = ("a", "a", "b", "b")
</list>
<trial mytrial>
/ ontrialbegin = [if(list.mylist.nextvalue=="a") trial.mytrial.insertstimulusframe(text.a,1);]
/ ontrialbegin = [if(list.mylist.nextvalue=="b") trial.mytrial.insertstimulusframe(text.b,1);]
/ validresponse = (57)
/ ontrialend = [trial.mytrial.resetstimulusframes();]
</trial>
<block myblock>
/ trials = [1-4=mytrial]
</block>
<data>
/ columns = [trialnum trialcode stimulusitem stimulusitem]
/ separatefiles = true
</data>