If you own, merged or bought an old account that has Competitor's Tabard, and transferred the character to a new battle.net account.

Then follow this exploit to own your Competitor's Tabard on main and alts, including the achievement Competitor's Tabard.
1- Go to legion Dalaran, and find Elizabeth Ross .

2- Talk to her.
2.1 - Buy Competitor's Tabard

She also sell/restore all the Tabards, check the one you are missing!

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