Your post is a little confusing, but this is what I pulled from it:
Junior employees are being asked to work doubles on preload from your sort and you're being skipped. That's an open and shut seniority violation; if you can, stick around and let the preload supervisor know you want to work and that less senior guys are being asked. If you're skipped again, include that in the grievance (so they can't complain about being blind sided) and ask to be paid for the hours worked (including OT) by one of the junior guys.
In the hub, supervisors tend to play favorites for two reasons: 1) there's no central "extra work/doubles list" being maintained by the sort managers (and if there is, finding it will take a miracle) and it's more convenient for supervisors to text/call the employees they are already familiar with and they know want to work or 2) they're blatantly playing favorites with the guys that'll come in load @ 500 pph in the outbound, etc.
Personally, as a steward, I prefer that the member go ahead and either personally (or ask me to) speak with the supervisor in question and remind them of the seniority order (along with providing the senior member's name/phone number) instead of coming in without being called and bumping someone who made arrangements to work that day. Extra work under my supplement is offered either via an extra work list (I would prefer this, personally) and/or established local practices, which includes supervisors have an "informal" list of people they want to work in seniority order.