How do I add previous flights to United?

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Tags

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2. #enter
3. #number
4. #ticket
5. #mileageplus
6. #first
7. #missing
8. #found
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10. #flight
11. #account
12. #request
13. #miles
14. #which
15. #boarding
16. #united
17. #credit
18. #partner
19. #receipt