Last year, when boarding the first leg of a three-leg ticket which had two checked bags per person, the airline (United) check-in attendant did not recognize that the second bag was already included and forced me to pay $100 for the second bag (just for the first leg). They instructed me to pay the fee then, and later follow up with the other airline (Turkish Airlines) for a refund. That turned out to be a run-around culminating in United credit (which I couldn't even use because I did not purchase the ticket from them).
This year I have a similar flight, and based on reports from others with the same route I expect the same confusion. What can I do to prepare and prove to the attendants that their computer is wrong and the ticket already includes the second bag?
Unfortunately I don't have any form of documentation which shows the ticket along with the number of checked bags. The travel agency repeatedly confirmed that all three legs have two checked bags each, and the agent did provide this excerpt:
UPTO50LB/23KG AND UPTO62LI/158LCM UPTO50LB/23KG AND UPTO62LI/158LCM
Is that likely to be enough?
Update: the above was not enough, nor a print-out from the Turkish Airlines confirmation page which showed two checked bags for the United flight. I was forced to pay again. The attendant said that the only way I would be allowed to check my second bag for free is if I had documentation from United Airlines which confirmed that I had two checked bags included in the ticket.
Some more information which I think might be useful from an answer that was deleted:
So far I have not found a way to avoid this unfair charge. All I have right now is a list of things which don't work:
Presenting documentation which shows that the bags are included in the ticket base fare. This includes excerpts from the GDS or from the reservation page of other airlines. Documentation which does not come from the airline (United) will not be considered.
Following the attendant's advice and seeking a refund from the airline who sold the ticket. Turkish Airlines will state that United was wrong to charge you and send you back to United.
Trying to get a refund from United online. One option which will be presented is a misleadingly-named "instant refund", which will give you credit for that airline, which you will not be able to use if you don't purchase tickets from them. I did not receive a response after seeking an appeal for the "instant refund".
The attendant's explanation was that, per United policy, regardless of what's in the ticket or reservation, it is United's policy to always allow only one free checked bag, unless the ticket was purchased directly from United Airlines.