Post Undeleted by Dennis
2 added 12 characters in body
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Jelly, 35 bytes

żŒsżŒsQ€

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How it works

żŒsQ€  Main link. Argument: s (string)

 Œs    Yield s with swapped case.
ż      Zip s with the result.
   Q€  Unique each; deduplicate each pair of characters.

Jelly, 3 bytes

żŒs

Try it online!

Jelly, 5 bytes

żŒsQ€

Try it online!

How it works

żŒsQ€  Main link. Argument: s (string)

 Œs    Yield s with swapped case.
ż      Zip s with the result.
   Q€  Unique each; deduplicate each pair of characters.
    Post Deleted by Dennis
1
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Jelly, 3 bytes

żŒs

Try it online!