over filled oil

Hi guys had my car fully services around 2 months ago however I checked my oil a few weeks ago and see it was overfull by alot so I did the oil change this morning I've not topped it up since work was done and no coolent loss and no water in oil so rules out head gasket the mechanic has done the job because my new sump washer and filter was on so how can I have around 7 liters in my engine? ?? I'm running on svo but I can't see it mixing :/ help

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