Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

@mixin hold-once($name) {
  // if placeholder not in declared ones
  @if not index($PLACEHOLDER__DECLARED, $name) {
    // set placeholder as declared
    $PLACEHOLDER__DECLARED: append($PLACEHOLDER__DECLARED, $name)!global;
    
    // declare placeholder
    %#{$name} {
      @content;
    }
  }
}
$PLACEHOLDER__DECLARED: ()!default;

%foo {
  foo: 1;
}
%foo {
  foo: 2;
}

@include hold-once('bar') {
  bar: 1;
}
@include hold-once('bar') {
  bar: 2;
}
@include hold-once('bar') {
  bar: 3;
}

.foo { @extend %foo }
.bar { @extend %bar }

SassMeister-output.css

.foo {
  foo: 1;
}

.foo {
  foo: 2;
}

.bar {
  bar: 1;
}