Permanent engine damage on a modern motor is unlikely. In the old days before good crankcase venting, pressure could be put on rubber engine seals, especially in very cold weather.
Nowadays fewer internal/external seals are used and most modern motors (like the XT) have good crankcase pressure venting.
Normally, on most bikes when oil is overfilled, the oil will have to go somewhere. At higher RPM's it can be pumped into the air box on many bikes. Most vent from the crankcase into the air box. Excess oil can then be drained from the air box but often times the air filter will become soaked with oil. A foam filter can simply be washed. A paper filter will have to be replaced.
Too much oil means much more friction or drag on the crank and internal moving parts. Too much oil pressure can also cause oil burning, as the excess oil level and pressure forces it past piston rings.
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