Happens because you're actually having <surveypage grouppage> run twice:
(1) First time via
<survey selectionphase>
/ pages = [1=grouppage]
[...]
</survey>
(2) Second time via
<block showchoices>
/ trials = [1 = grouppage; 2 = summary]
</block>
Note that the <survey> element is just a special kind of <block> element. Similarly, the <surveypage> element is just a special type of <trial> element. To fix things, eliminate grouppage from <block showchoices> and set up an <expt> element to run <survey selectionphase> and <block showchoices> in the correct order:
<expt>
/ blocks = [1=selectionphase; 2=showchoices]
[...]
</expt>
<survey selectionphase>
/ pages = [1=grouppage]
[...]
</survey>
<block showchoices>
/ trials = [1 = summary]
</block>
Regards,
~Dave