One way strikes me: just create a variable that gets set to true once the import is called. For example:

{
  my %imported;
  sub import {
    my( $class, @exports ) = @_;

    return if $imported{$class}++;
    # rest of your import here.
  }
}
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By closing on a lexical like this, it won't show up in your namespace. Not that it probably matters, but it may be a good trick to avoid this module from exporting "globals" from other packages upwards.