+1. This is common on mowers where the fuel is ABOVE the carb. You MUST turn the fuel valve OFF after each use. If it has an "automatic" fuel shut off, then install a manual version so that you can be sure the fuel is really OFF.
Easy to check for this situation. Remove the dip stick and check the oil level. If it seems higher than normal, then the extra fluid is gasoline. Besides the engine repairs, you'll also need to change your engine oil.