the extract has been taken from CIE MAY/JUNE 2009 timetable..:
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Timetable clashes
If a candidate is entered for two papers that are timetabled for the
same session, the rules depend on the total duration of the two
papers.
If the total duration is 3 hours and 45 minutes or less:
both papers should be taken in the same session with a supervised
break between papers. You do not need to notify CIE about this.
If the total duration is more than 3 hours and 45 minutes:
you must submit a Timetable Deviation Form (INT/E24) to CIE.
Please note that whenever there is a timetable deviation, candidates
starting an examination at any time that is not the scheduled time,
must be kept isolated from candidates who have already taken the
paper. For further information about timetable clashes, please refer to
the appropriate section of the CIE Handbook for Centres 2009.
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hope it clears your doubt..
if not do as the moderator angell said ...