Ive had a new head gasket fitted, but the oil and water are still mixing after only 24 hours of the job being done

. Someone said to me it is possible that the head gasket has a crack in it , but then i was also told it is
very rare that they crack. The mechaninc has now fitted a new headgasket twice but the problem still persists. Does anyone have any idea what might me wrong ?
Thanks
Paul