Step 1: you need a bike that you want to duplicate.
Step 2: you will also need the MULE, once you have the MULE you will need to go and collect it from pegasus and park it in from of your garage.
Step 3: Once you have parked the MULE outside you garage (works best with 400k apartment building) pop all the tires and make sure the back is facing the garage and the front wheels are half on the white brick and half on the black slabs.
Step 4: Go into your garage and get the bike you want to duplicate.
Step 5: Drive out and you should smash into the back of the MULE, a few seconds later you should glitch out under the MULE with your bike and this should be the duplicated one, you will know you have done this correctly if there is another bike with a tracker on it.
Step 6: Get on the duplicated bike an drive to LS Customs. Once there you will now need to go on to Legendary Motorsport and choose a car that you want to increase the value of the bike with (adder makes the bike worth 600k). Once you have bought a car, call up you mechanic and park you front wheel on the bike just over the line at the entrance of LS Customs. As soon as your mechanic answer choose the apartment that you have stored the vehicle you bought
Step 7: Once you are in LS Customs choose the vehicle you have bought on the list that comes up from the mechanic. Now press Right on the D-Pad and go to sell, Press A once and then wait for the Orange loading circle to appear and then disappear.
Step 8: after it has disappeared leave LS customs and drive back to your garage and your vehicle is now worth an increased value, if you used the adder you can now sell it for 600k. This is perfect if you can do the duplication glitch with the bike and make millions of GTA V very quickly

Thank You and Hope this works for you guys!
I personally found this glitch as well, but I took multiple glitches and figured out a solo version.

Post for any questions or problems!