When you spawn in the projectile, send in the “Self” variable as the instigator. Then in the projectile blueprint, add a check to your “on overlap” to make sure that the other actor is not the same as the instigator (using “get Instigator”). This worked for me, and I have my collision preset to overlap all dynamic.
If the problem is with the player’s collision mesh instead of the projectile, you can set a variable to the spawned actor, and then check your own collision capsule against that variable to see if the touching projectile is equal to the one the player shot.