How to prevent a overlay from being applied more than once?

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2

You could add a marker to nixpkgs to check if the overlay has been applied already:

let
  overlayFoo = super.lib.optionalAttrs (!super.__fooHasBeenApplied or false) {
    __fooHasBeenApplied = true;
    # …
  };
in
#…

It is probably simpler to get rid of the composeExtensions though.

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